PMCC PMCC

Search tips
Search criteria

Advanced
Results 1-25 (1206078)

Clipboard (0)
None

Related Articles

1.  Functional Coupling of Ryanodine Receptors to KCa Channels in Smooth Muscle Cells from Rat Cerebral Arteries  
The Journal of General Physiology  1999;113(2):229-238.
The relationship between Ca2+ release (“Ca2+ sparks”) through ryanodine-sensitive Ca2+ release channels in the sarcoplasmic reticulum and KCa channels was examined in smooth muscle cells from rat cerebral arteries. Whole cell potassium currents at physiological membrane potentials (−40 mV) and intracellular Ca2+ were measured simultaneously, using the perforated patch clamp technique and a laser two-dimensional (x–y) scanning confocal microscope and the fluorescent Ca2+ indicator, fluo-3. Virtually all (96%) detectable Ca2+ sparks were associated with the activation of a spontaneous transient outward current (STOC) through KCa channels. A small number of sparks (5 of 128) were associated with currents smaller than 6 pA (mean amplitude, 4.7 pA, at −40 mV). Approximately 41% of STOCs occurred without a detectable Ca2+ spark. The amplitudes of the Ca2+ sparks correlated with the amplitudes of the STOCs (regression coefficient 0.8; P < 0.05). The half time of decay of Ca2+ sparks (56 ms) was longer than the associated STOCs (9 ms). The mean amplitude of the STOCs, which were associated with Ca2+ sparks, was 33 pA at −40 mV. The mean amplitude of the “sparkless” STOCs was smaller, 16 pA. The very significant increase in KCa channel open probability (>104-fold) during a Ca2+ spark is consistent with local Ca2+ during a spark being in the order of 1–100 μM. Therefore, the increase in fractional fluorescence (F/Fo) measured during a Ca2+ spark (mean 2.04 F/Fo or ∼310 nM Ca2+) appears to significantly underestimate the local Ca2+ that activates KCa channels. These results indicate that the majority of ryanodine receptors that cause Ca2+ sparks are functionally coupled to KCa channels in the surface membrane, providing direct support for the idea that Ca2+ sparks cause STOCs.
PMCID: PMC2223357  PMID: 9925821
Ca2+ sparks; ryanodine receptor; sarcoplasmic reticulum; potassium currents; smooth muscle
2.  Dynamics of Signaling between Ca2+ Sparks and Ca2+- Activated K+ Channels Studied with a Novel Image-Based Method for Direct Intracellular Measurement of Ryanodine Receptor Ca2+ Current 
The Journal of General Physiology  2000;116(6):845-864.
Ca2+ sparks are highly localized cytosolic Ca2+ transients caused by a release of Ca2+ from the sarcoplasmic reticulum via ryanodine receptors (RyRs); they are the elementary events underlying global changes in Ca2+ in skeletal and cardiac muscle. In smooth muscle and some neurons, Ca2+ sparks activate large conductance Ca2+-activated K+ channels (BK channels) in the spark microdomain, causing spontaneous transient outward currents (STOCs) that regulate membrane potential and, hence, voltage-gated channels. Using the fluorescent Ca2+ indicator fluo-3 and a high speed widefield digital imaging system, it was possible to capture the total increase in fluorescence (i.e., the signal mass) during a spark in smooth muscle cells, which is the first time such a direct approach has been used in any system. The signal mass is proportional to the total quantity of Ca2+ released into the cytosol, and its rate of rise is proportional to the Ca2+ current flowing through the RyRs during a spark (ICa(spark)). Thus, Ca2+ currents through RyRs can be monitored inside the cell under physiological conditions. Since the magnitude of ICa(spark) in different sparks varies more than fivefold, Ca2+ sparks appear to be caused by the concerted opening of a number of RyRs. Sparks with the same underlying Ca2+ current cause STOCs, whose amplitudes vary more than threefold, a finding that is best explained by variability in coupling ratio (i.e., the ratio of RyRs to BK channels in the spark microdomain). The time course of STOC decay is approximated by a single exponential that is independent of the magnitude of signal mass and has a time constant close to the value of the mean open time of the BK channels, suggesting that STOC decay reflects BK channel kinetics, rather than the time course of [Ca2+] decline at the membrane. Computer simulations were carried out to determine the spatiotemporal distribution of the Ca2+ concentration resulting from the measured range of ICa(spark). At the onset of a spark, the Ca2+ concentration within 200 nm of the release site reaches a plateau or exceeds the [Ca2+]EC50 for the BK channels rapidly in comparison to the rate of rise of STOCs. These findings suggest a model in which the BK channels lie close to the release site and are exposed to a saturating [Ca2+] with the rise and fall of the STOCs determined by BK channel kinetics. The mechanism of signaling between RyRs and BK channels may provide a model for Ca2+ action on a variety of molecular targets within cellular microdomains.
PMCID: PMC2231814  PMID: 11099351
widefield digital microscope; sarcoplasmic reticulum; microdomain; STOC; smooth muscle release
3.  Effects of Tetracaine on Voltage-activated Calcium Sparks in Frog Intact Skeletal Muscle Fibers 
The Journal of General Physiology  2006;127(3):291-307.
The properties of Ca2+ sparks in frog intact skeletal muscle fibers depolarized with 13 mM [K+] Ringer's are well described by a computational model with a Ca2+ source flux of amplitude 2.5 pA (units of current) and duration 4.6 ms (18 °C; Model 2 of Baylor et al., 2002). This result, in combination with the values of single-channel Ca2+ current reported for ryanodine receptors (RyRs) in bilayers under physiological ion conditions, 0.5 pA (Kettlun et al., 2003) to 2 pA (Tinker et al., 1993), suggests that 1–5 RyR Ca2+ release channels open during a voltage-activated Ca2+ spark in an intact fiber. To distinguish between one and greater than one channel per spark, sparks were measured in 8 mM [K+] Ringer's in the absence and presence of tetracaine, an inhibitor of RyR channel openings in bilayers. The most prominent effect of 75–100 μM tetracaine was an approximately sixfold reduction in spark frequency. The remaining sparks showed significant reductions in the mean values of peak amplitude, decay time constant, full duration at half maximum (FDHM), full width at half maximum (FWHM), and mass, but not in the mean value of rise time. Spark properties in tetracaine were simulated with an updated spark model that differed in minor ways from our previous model. The simulations show that (a) the properties of sparks in tetracaine are those expected if tetracaine reduces the number of active RyR Ca2+ channels per spark, and (b) the single-channel Ca2+ current of an RyR channel is ≤1.2 pA under physiological conditions. The results support the conclusion that some normal voltage-activated sparks (i.e., in the absence of tetracaine) are produced by two or more active RyR Ca2+ channels. The question of how the activation of multiple RyRs is coordinated is discussed.
doi:10.1085/jgp.200509477
PMCID: PMC2151506  PMID: 16505149
4.  Impact of subarachnoid hemorrhage on local and global calcium signaling in cerebral artery myocytes 
Acta neurochirurgica. Supplement  2011;110(Pt 1):145-150.
Summary
Background
Ca2+ signaling mechanisms are crucial for proper regulation of vascular smooth muscle contractility and vessel diameter. In cerebral artery myocytes, a rise in global cytosolic Ca2+ concentration ([Ca2+]i) causes contraction while an increase in local Ca2+ release events from the sarcoplasmic reticulum (Ca2+ sparks) leads to increased activity of large-conductance Ca2+-activated (BK) K+ channels, hyperpolarization and relaxation. Here, we examined the impact of SAH on Ca2+ spark activity and [Ca2+]i in cerebral artery myocytes following SAH.
Methods
A rabbit double injection SAH model was used in this study. Five days after the initial intracisternal injection of whole blood, small diameter cerebral arteries were dissected from the brain for study. For simultaneous measurement of arterial wall [Ca2+]i and diameter, vessels were cannulated and loaded with the ratiometric Ca2+ indicator fura-2. For measurement of Ca2+ sparks, individual myocytes were enzymatically isolated from cerebral arteries and loaded with the Ca2+ indicator fluo-4. Sparks were visualized using laser scanning confocal microscopy.
Results
Arterial wall [Ca2+]i was significantly elevated and greater levels of myogenic tone developed in arteries isolated from SAH animals compared with arteries isolated from healthy animals. The L-type voltage-dependent Ca2+ channel (VDCC) blocker nifedipine attenuated increases in [Ca2+]i and tone in both groups suggesting increased VDCC activity following SAH. Membrane potential measurement using intracellular microelectrodes revealed significant depolarization of vascular smooth muscle following SAH. Further, myocytes from SAH animals exhibited significantly reduced Ca2+ spark frequency (~50%).
Conclusions
Our findings suggest decreased Ca2+ spark frequency leads to reduced BK channel activity in cerebral artery myocytes following SAH. This results in membrane potential depolarization, increased VDCC activity, elevated [Ca2+]i and decreased vessel diameter. We propose this mechanism of enhanced cerebral artery myocyte contractility may contribute to decreased cerebral blood flow and development of neurological deficits in SAH patients.
doi:10.1007/978-3-7091-0353-1_25
PMCID: PMC3057755  PMID: 21116930
Ca2+ channels; K+ channels; Ca2+ sparks; vascular smooth muscle; vasospasm
5.  A Close Association of RyRs with Highly Dense Clusters of Ca2+-activated Cl− Channels Underlies the Activation of STICs by Ca2+ Sparks in Mouse Airway Smooth Muscle 
The Journal of General Physiology  2008;132(1):145-160.
Ca2+ sparks are highly localized, transient releases of Ca2+ from sarcoplasmic reticulum through ryanodine receptors (RyRs). In smooth muscle, Ca2+ sparks trigger spontaneous transient outward currents (STOCs) by opening nearby clusters of large-conductance Ca2+-activated K+ channels, and also gate Ca2+-activated Cl− (Cl(Ca)) channels to induce spontaneous transient inward currents (STICs). While the molecular mechanisms underlying the activation of STOCs by Ca2+ sparks is well understood, little information is available on how Ca2+ sparks activate STICs. In the present study, we investigated the spatial organization of RyRs and Cl(Ca) channels in spark sites in airway myocytes from mouse. Ca2+ sparks and STICs were simultaneously recorded, respectively, with high-speed, widefield digital microscopy and whole-cell patch-clamp. An image-based approach was applied to measure the Ca2+ current underlying a Ca2+ spark (ICa(spark)), with an appropriate correction for endogenous fixed Ca2+ buffer, which was characterized by flash photolysis of NPEGTA. We found that ICa(spark) rises to a peak in 9 ms and decays with a single exponential with a time constant of 12 ms, suggesting that Ca2+ sparks result from the nonsimultaneous opening and closure of multiple RyRs. The onset of the STIC lags the onset of the ICa(spark) by less than 3 ms, and its rising phase matches the duration of the ICa(spark). We further determined that Cl(Ca) channels on average are exposed to a [Ca2+] of 2.4 μM or greater during Ca2+ sparks. The area of the plasma membrane reaching this level is <600 nm in radius, as revealed by the spatiotemporal profile of [Ca2+] produced by a reaction-diffusion simulation with measured ICa(spark). Finally we estimated that the number of Cl(Ca) channels localized in Ca2+ spark sites could account for all the Cl(Ca) channels in the entire cell. Taken together these results lead us to propose a model in which RyRs and Cl(Ca) channels in Ca2+ spark sites localize near to each other, and, moreover, Cl(Ca) channels concentrate in an area with a radius of ∼600 nm, where their density reaches as high as 300 channels/μm2. This model reveals that Cl(Ca) channels are tightly controlled by Ca2+ sparks via local Ca2+ signaling.
doi:10.1085/jgp.200709933
PMCID: PMC2442178  PMID: 18591421
6.  Abnormal Ca2+ Spark/STOC Coupling in Cerebral Artery Smooth Muscle Cells of Obese Type 2 Diabetic Mice 
PLoS ONE  2013;8(1):e53321.
Diabetes is a major risk factor for stroke. However, the molecular mechanisms involved in cerebral artery dysfunction found in the diabetic patients are not completely elucidated. In cerebral artery smooth muscle cells (CASMCs), spontaneous and local increases of intracellular Ca2+ due to the opening of ryanodine receptors (Ca2+ sparks) activate large conductance Ca2+-activated K+ (BK) channels that generate spontaneous transient outward currents (STOCs). STOCs have a key participation in the control of vascular myogenic tone and blood pressure. Our goal was to investigate whether alterations in Ca2+ spark and STOC activities, measured by confocal microscopy and patch-clamp technique, respectively, occur in isolated CASMCs of an experimental model of type-2 diabetes (db/db mouse). We found that mean Ca2+ spark amplitude, duration, size and rate-of-rise were significantly smaller in Fluo-3 loaded db/db compared to control CASMCs, with a subsequent decrease in the total amount of Ca2+ released through Ca2+ sparks in db/db CASMCs, though Ca2+ spark frequency remained. Interestingly, the frequency of large-amplitude Ca2+ sparks was also significantly reduced in db/db cells. In addition, the frequency and amplitude of STOCs were markedly reduced at all voltages tested (from −50 to 0 mV) in db/db CASMCs. The latter correlates with decreased BK channel β1/α subunit ratio found in db/db vascular tissues. Taken together, Ca2+ spark alterations lead to inappropriate BK channels activation in CASMCs of db/db mice and this condition is aggravated by the decrease in the BK β1 subunit/α subunit ratio which underlies the significant reduction of Ca2+ spark/STOC coupling in CASMCs of diabetic animals.
doi:10.1371/journal.pone.0053321
PMCID: PMC3536748  PMID: 23301060
7.  Genetic ablation of caveolin-1 modifies Ca2+ spark coupling in murine arterial smooth muscle cells 
L-type, voltage-dependent calcium (Ca2+) channels, ryanodine-sensitive Ca2+ release (RyR) channels, and large-conductance Ca2+-activated potassium (KCa) channels comprise a functional unit that regulates smooth muscle contractility. Here, we investigated whether genetic ablation of caveolin-1 (cav-1), a caveolae protein, alters Ca2+ spark to KCa channel coupling and Ca2+ spark regulation by voltage-dependent Ca2+ channels in murine cerebral artery smooth muscle cells. Caveolae were abundant in the sarcolemma of control (cav-+/+) cells but were not observed in cav-1-deficient (cav-1−/−) cells. Ca2+ spark and transient KCa current frequency were approximately twofold higher in cav-1−/− than in cav-1+/+ cells. Although voltage-dependent Ca2+ current density was similar in cav-1+/+ and cav-1−/− cells, diltiazem and Cd2+, voltage-dependent Ca2+ channel blockers, reduced transient KCa current frequency to ∼55% of control in cav-1+/+ cells but did not alter transient KCa current frequency in cav-1−/− cells. Furthermore, although KCa channel density was elevated in cav-1−/− cells, transient KCa current amplitude was similar to that in cav-1+/+ cells. Higher Ca2+ spark frequency in cav-1−/− cells was not due to elevated intracellular Ca2+ concentration, sarcoplasmic reticulum Ca2+ load, or nitric oxide synthase activity. Similarly, Ca2+ spark amplitude and spread, the percentage of Ca2+ sparks that activated a transient KCa current, the amplitude relationship between sparks and transient KCa currents, and KCa channel conductance and apparent Ca2+ sensitivity were similar in cav-1+/+ and cav-1−/− cells. In summary, cav-1 ablation elevates Ca2+ spark and transient KCa current frequency, attenuates the coupling relationship between voltage-dependent Ca2+ channels and RyR channels that generate Ca2+ sparks, and elevates KCa channel density but does not alter transient KCa current activation by Ca2+ sparks. These findings indicate that cav-1 is required for physiological Ca2+ spark and transient KCa current regulation in cerebral artery smooth muscle cells.
doi:10.1152/ajpheart.01226.2005
PMCID: PMC1698957  PMID: 16428350
ryanodine-sensitive Ca2+ release channel; large-conductance Ca2+-activated potassium channel; caveolae; voltage-dependent Ca2+ channel
8.  KCa channel insensitivity to Ca2+ sparks underlies fractional uncoupling in newborn cerebral artery smooth muscle cells 
In smooth muscle cells, localized intracellular Ca2+ transients, termed “Ca2+ sparks,” activate several large-conductance Ca2+-activated K+ (KCa) channels, resulting in a transient KCa current. In some smooth muscle cell types, a significant proportion of Ca2+ sparks do not activate KCa channels. The goal of this study was to explore mechanisms that underlie fractional Ca2+ spark-KCa channel coupling. We investigated whether membrane depolarization or ryanodine-sensitive Ca2+ release (RyR) channel activation modulates coupling in newborn (1- to 3-day-old) porcine cerebral artery myocytes. At steady membrane potentials of −40, 0, and +40 mV, mean transient KCa current frequency was ∼0.18, 0.43, and 0.26 Hz and KCa channel activity [number of KCa channels activated by Ca2+ sparks × open probability of KCa channels at peak of Ca2+ sparks (NPo)] at the transient KCa current peak was ∼4, 12, and 24, respectively. Depolarization between −40 and +40 mV increased KCa channel sensitivity to Ca2+ sparks and elevated the percentage of Ca2+ sparks that activated a transient KCa current from 59 to 86%. In a Ca2+-free bath solution or in diltiazem, a voltage-dependent Ca2+ channel blocker, steady membrane depolarization between −40 and +40 mV increased transient KCa current frequency up to ∼1.6-fold. In contrast, caffeine (10 μM), an RyR channel activator, increased mean transient KCa current frequency but did not alter Ca2+ spark-KCa channel coupling. These data indicate that coupling is increased by mechanisms that elevate KCa channel sensitivity to Ca2+ sparks, but not by RyR channel activation. Overall, KCa channel insensitivity to Ca2+ sparks is a prominent factor underlying fractional Ca2+ spark uncoupling in newborn cerebral artery myocytes.
doi:10.1152/ajpheart.01308.2005
PMCID: PMC1752210  PMID: 16603686
ryanodine-sensitive calcium release channel; calcium-activated potassium channel; membrane potential
9.  The Influence of Sarcoplasmic Reticulum Ca2+ Concentration on Ca2+ Sparks and Spontaneous Transient Outward Currents in Single Smooth Muscle Cells  
The Journal of General Physiology  1999;113(2):215-228.
Localized, transient elevations in cytosolic Ca2+, known as Ca2+ sparks, caused by Ca2+ release from sarcoplasmic reticulum, are thought to trigger the opening of large conductance Ca2+-activated potassium channels in the plasma membrane resulting in spontaneous transient outward currents (STOCs) in smooth muscle cells. But the precise relationships between Ca2+ concentration within the sarcoplasmic reticulum and a Ca2+ spark and that between a Ca2+ spark and a STOC are not well defined or fully understood. To address these problems, we have employed two approaches using single patch-clamped smooth muscle cells freshly dissociated from toad stomach: a high speed, wide-field imaging system to simultaneously record Ca2+ sparks and STOCs, and a method to simultaneously measure free global Ca2+ concentration in the sarcoplasmic reticulum ([Ca2+]SR) and in the cytosol ([Ca2+]CYTO) along with STOCs. At a holding potential of 0 mV, cells displayed Ca2+ sparks and STOCs. Ca2+ sparks were associated with STOCs; the onset of the sparks coincided with the upstroke of STOCs, and both had approximately the same decay time. The mean increase in [Ca2+]CYTO at the time and location of the spark peak was ∼100 nM above a resting concentration of ∼100 nM. The frequency and amplitude of spontaneous Ca2+ sparks recorded at −80 mV were unchanged for a period of 10 min after removal of extracellular Ca2+ (nominally Ca2+-free solution with 50 μM EGTA), indicating that Ca2+ influx is not necessary for Ca2+sparks. A brief pulse of caffeine (20 mM) elicited a rapid decrease in [Ca2+]SR in association with a surge in [Ca2+]CYTO and a fusion of STOCs, followed by a fast restoration of [Ca2+]CYTO and a gradual recovery of [Ca2+]SR and STOCs. The return of global [Ca2+]CYTO to rest was an order of magnitude faster than the refilling of the sarcoplasmic reticulum with Ca2+. After the global [Ca2+]CYTO was fully restored, recovery of STOC frequency and amplitude were correlated with the level of [Ca2+]SR, even though the time for refilling varied greatly. STOC frequency did not recover substantially until the [Ca2+]SR was restored to 60% or more of resting levels. At [Ca2+]SR levels above 80% of rest, there was a steep relationship between [Ca2+]SR and STOC frequency. In contrast, the relationship between [Ca2+]SR and STOC amplitude was linear. The relationship between [Ca2+]SR and the frequency and amplitude was the same for Ca2+ sparks as it was for STOCs. The results of this study suggest that the regulation of [Ca2+]SR might provide one mechanism whereby agents could govern Ca2+ sparks and STOCs. The relationship between Ca2+ sparks and STOCs also implies a close association between a sarcoplasmic reticulum Ca2+ release site and the Ca2+-activated potassium channels responsible for a STOC.
PMCID: PMC2223361  PMID: 9925820
Ca2+ spark; spontaneous transient outward current; Mag-fura-2; [Ca2+]SR; ryanodine receptor
10.  Role of Ryanodine Receptor Subtypes in Initiation and Formation of Calcium Sparks in Arterial Smooth Muscle: Comparison with Striated Muscle 
Calcium sparks represent local, rapid, and transient calcium release events from a cluster of ryanodine receptors (RyRs) in the sarcoplasmic reticulum. In arterial smooth muscle cells (SMCs), calcium sparks activate calcium-dependent potassium channels causing decrease in the global intracellular [Ca2+] and oppose vasoconstriction. This is in contrast to cardiac and skeletal muscle, where spatial and temporal summation of calcium sparks leads to global increases in intracellular [Ca2+] and myocyte contraction. We summarize the present data on local RyR calcium signaling in arterial SMCs in comparison to striated muscle and muscle-specific differences in coupling between L-type calcium channels and RyRs. Accordingly, arterial SMC Cav1.2 L-type channels regulate intracellular calcium stores content, which in turn modulates calcium efflux though RyRs. Downregulation of RyR2 up to a certain degree is compensated by increased SR calcium content to normalize calcium sparks. This indirect coupling between Cav1.2 and RyR in arterial SMCs is opposite to striated muscle, where triggering of calcium sparks is controlled by rapid and direct cross-talk between Cav1.1/Cav1.2 L-type channels and RyRs. We discuss the role of RyR isoforms in initiation and formation of calcium sparks in SMCs and their possible molecular binding partners and regulators, which differ compared to striated muscle.
doi:10.1155/2009/135249
PMCID: PMC2793424  PMID: 20029633
11.  Detection, Properties, and Frequency of Local Calcium Release from the Sarcoplasmic Reticulum in Teleost Cardiomyocytes 
PLoS ONE  2011;6(8):e23708.
Calcium release from the sarcoplasmic reticulum (SR) plays a central role in the regulation of cardiac contraction and rhythm in mammals and humans but its role is controversial in teleosts. Since the zebrafish is an emerging model for studies of cardiovascular function and regeneration we here sought to determine if basic features of SR calcium release are phylogenetically conserved. Confocal calcium imaging was used to detect spontaneous calcium release (calcium sparks and waves) from the SR. Calcium sparks were detected in 16 of 38 trout atrial myocytes and 6 of 15 ventricular cells. The spark amplitude was 1.45±0.03 times the baseline fluorescence and the time to half maximal decay of sparks was 27±3 ms. Spark frequency was 0.88 sparks µm−1 min−1 while calcium waves were 8.5 times less frequent. Inhibition of SR calcium uptake reduced the calcium transient (F/F0) from 1.77±0.17 to 1.12±0.18 (p = 0.002) and abolished calcium sparks and waves. Moreover, elevation of extracellular calcium from 2 to 10 mM promoted early and delayed afterdepolarizations (from 0.6±0.3 min−1 to 8.1±2.0 min−1, p = 0.001), demonstrating the ability of SR calcium release to induce afterdepolarizations in the trout heart. Calcium sparks of similar width and duration were also observed in zebrafish ventricular myocytes. In conclusion, this is the first study to consistently report calcium sparks in teleosts and demonstrate that the basic features of calcium release through the ryanodine receptor are conserved, suggesting that teleost cardiac myocytes is a relevant model to study the functional impact of abnormal SR function.
doi:10.1371/journal.pone.0023708
PMCID: PMC3163583  PMID: 21897853
12.  Identification and Characterization of Calcium Sparks in Cardiomyocytes Derived from Human Induced Pluripotent Stem Cells 
PLoS ONE  2013;8(2):e55266.
Introduction
Ca2+ spark constitutes the elementary units of cardiac excitation-contraction (E-C) coupling in mature cardiomyocytes. Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes are known to have electrophysiological properties similar to mature adult cardiomyocytes. However, it is unclear if they share similar calcium handling property. We hypothesized that Ca2+ sparks in human induced pluripotent stem cell (hiPSCs)-derived cardiomyocytes (hiPSC-CMs) may display unique structural and functional properties than mature adult cardiomyocytes.
Methods and results
Ca2+ sparks in hiPSC-CMs were recorded with Ca2+ imaging assay with confocal laser scanning microscopy. Those sparks were stochastic with a tendency of repetitive occurrence at the same site. Nevertheless, the spatial-temporal properties of Ca2+ spark were analogous to that of adult CMs. Inhibition of L-type Ca2+ channels by nifedipine caused a 61% reduction in calcium spark frequency without affecting amplitude of those sparks and magnitude of caffeine releasable sarcoplasmic reticulum (SR) Ca2+ content. In contrast, high extracellular Ca2+ and ryanodine increased the frequency, full width at half maximum (FWHM) and full duration at half maximum (FDHM) of spontaneous Ca2+ sparks.
Conclusions
For the first time, spontaneous Ca2+ sparks were detected in hiPSC-CMs. The Ca2+ sparks are predominately triggered by L-type Ca2+ channels mediated Ca2+ influx, which is comparable to sparks detected in adult ventricular myocytes in which cardiac E-C coupling was governed by a Ca2+-induced Ca2+ release (CICR) mechanism. However, focal repetitive sparks originated from the same intracellular organelle could reflect an immature status of the hiPSC-CMs.
doi:10.1371/journal.pone.0055266
PMCID: PMC3567046  PMID: 23408964
13.  Comparison of Simulated and Measured Calcium Sparks in Intact Skeletal Muscle Fibers of the Frog 
The Journal of General Physiology  2002;120(3):349-368.
Calcium sparks in frog intact skeletal muscle fibers were modeled as stereotypical events that arise from a constant efflux of Ca2+ from a point source for a fixed period of time (e.g., 2.5 pA of Ca2+ current for 4.6 ms; 18°C). The model calculates the local changes in the concentrations of free Ca2+ and of Ca2+ bound to the major intrinsic myoplasmic Ca2+ buffers (troponin, ATP, parvalbumin, and the SR Ca2+ pump) and to the Ca2+ indicator (fluo-3). A distinctive feature of the model is the inclusion of a binding reaction between fluo-3 and myoplasmic proteins, a process that strongly affects fluo-3′s Ca2+-reaction kinetics, its apparent diffusion constant, and hence the morphology of sparks. ΔF/F (the change in fluo-3′s fluorescence divided by its resting fluorescence) was estimated from the calculated changes in fluo-3 convolved with the microscope point-spread function. To facilitate comparisons with measured sparks, noise and other sources of variability were included in a random repetitive fashion to generate a large number of simulated sparks that could be analyzed in the same way as the measured sparks. In the initial simulations, the binding of Ca2+ to the two regulatory sites on troponin was assumed to follow identical and independent binding reactions. These simulations failed to accurately predict the falling phase of the measured sparks. A second set of simulations, which incorporated the idea of positive cooperativity in the binding of Ca2+ to troponin, produced reasonable agreement with the measurements. Under the assumption that the single channel Ca2+ current of a ryanodine receptor (RYR) is 0.5–2 pA, the results suggest that 1–5 active RYRs generate an average Ca2+ spark in a frog intact muscle fiber.
doi:10.1085/jgp.20028620
PMCID: PMC2229517  PMID: 12198091
spark simulations; ryanodine receptors; fluo-3; excitation-contraction coupling; frog muscle
14.  Gender Differences in Myogenic Regulation along the Vascular Tree of the Gerbil Cochlea 
PLoS ONE  2011;6(9):e25659.
Regulation of cochlear blood flow is critical for hearing due to its exquisite sensitivity to ischemia and oxidative stress. Many forms of hearing loss such as sensorineural hearing loss and presbyacusis may involve or be aggravated by blood flow disorders. Animal experiments and clinical outcomes further suggest that there is a gender preference in hearing loss, with males being more susceptible. Autoregulation of cochlear blood flow has been demonstrated in some animal models in vivo, suggesting that similar to the brain, blood vessels supplying the cochlea have the ability to control flow within normal limits, despite variations in systemic blood pressure. Here, we investigated myogenic regulation in the cochlear blood supply of the Mongolian gerbil, a widely used animal model in hearing research. The cochlear blood supply originates at the basilar artery, followed by the anterior inferior cerebellar artery, and inside the inner ear, by the spiral modiolar artery and the radiating arterioles that supply the capillary beds of the spiral ligament and stria vascularis. Arteries from male and female gerbils were isolated and pressurized using a concentric pipette system. Diameter changes in response to increasing luminal pressures were recorded by laser scanning microscopy. Our results show that cochlear vessels from male and female gerbils exhibit myogenic regulation but with important differences. Whereas in male gerbils, both spiral modiolar arteries and radiating arterioles exhibited pressure-dependent tone, in females, only radiating arterioles had this property. Male spiral modiolar arteries responded more to L-NNA than female spiral modiolar arteries, suggesting that NO-dependent mechanisms play a bigger role in the myogenic regulation of male than female gerbil cochlear vessels.
doi:10.1371/journal.pone.0025659
PMCID: PMC3183064  PMID: 21980520
15.  Simulation of Calcium Sparks in Cut Skeletal Muscle Fibers of the Frog 
The Journal of General Physiology  2003;121(4):311-324.
Spark mass, the volume integral of ΔF/F, was investigated theoretically and with simulations. These studies show that the amount of Ca2+ bound to fluo-3 is proportional to mass times the total concentration of fluo-3 ([fluo-3T]); the proportionality constant depends on resting Ca2+ concentration ([Ca2+]R). In the simulation of a Ca2+ spark in an intact frog fiber with [fluo-3T] = 100 μM, fluo-3 captures approximately one-fourth of the Ca2+ released from the sarcoplasmic reticulum (SR). Since mass in cut fibers is several times that in intact fibers, both with similar values of [fluo-3T] and [Ca2+]R, it seems likely that SR Ca2+ release is larger in cut fiber sparks or that fluo-3 is able to capture a larger fraction of the released Ca2+ in cut fibers, perhaps because of reduced intrinsic Ca2+ buffering. Computer simulations were used to identify these and other factors that may underlie the differences in mass and other properties of sparks in intact and cut fibers. Our spark model, which successfully simulates calcium sparks in intact fibers, was modified to reflect the conditions of cut fiber measurements. The results show that, if the protein Ca2+-buffering power of myoplasm is the same as that in intact fibers, the Ca2+ source flux underlying a spark in cut fibers is 5–10 times that in intact fibers. Smaller source fluxes are required for less buffer. In the extreme case in which Ca2+ binding to troponin is zero, the source flux needs to be 3–5 times that in intact fibers. An increased Ca2+ source flux could arise from an increase in Ca2+ flux through one ryanodine receptor (RYR) or an increase in the number of active RYRs per spark, or both. These results indicate that the gating of RYRs, or their apparent single channel Ca2+ flux, is different in frog cut fibers—and, perhaps, in other disrupted preparations—than in intact fibers.
doi:10.1085/jgp.200308787
PMCID: PMC2217372  PMID: 12642597
spark mass; ryanodine receptors; excitation-contraction coupling; frog muscle
16.  TNF-α Dilates Cerebral Arteries via NAD(P)H Oxidase-Dependent Ca2+ Spark Activation 
Expression of Tumor Necrosis Factor alpha (TNF-α), a pleiotropic cytokine, is elevated during stroke and cerebral ischemia. TNF-α regulates arterial diameter, although mechanisms mediating this effect are unclear. Here, we tested the hypothesis that TNF-α regulates the diameter of resistance-size (∼150 μm diameter) cerebral arteries by modulating local and global intracellular calcium (Ca2+) signals in smooth muscle cells. Laser-scanning confocal imaging revealed that TNF-α increased Ca2+ spark and Ca2+ wave frequency, but reduced global Ca2+ concentration ([Ca2+]i) in smooth muscle cells of intact arteries. TNF-α elevated reactive oxygen species (ROS) in smooth muscle cells of intact arteries and this was prevented by apocynin or diphenyleneiodonium (DPI), NAD(P)H oxidase blockers, but was unaffected by inhibitors of other ROS generating enzymes. In voltage-clamped (-40 mV) cells, TNF-α increased the frequency and amplitude of Ca2+ spark-induced large-conductance Ca2+-activated potassium (KCa) channel transients ∼1.7-fold and ∼1.4-fold, respectively. TNF-α-induced transient KCa current activation was reversed by apocynin or MnTMPyP, a membrane-permeant antioxidant, and prevented by intracellular dialysis of catalase. TNF-α induced reversible and similar amplitude dilations in either endothelium-intact or-denuded pressurized (60 mm Hg) cerebral arteries. MnTMPyP, thapsigargin, a sarcoplasmic reticulum Ca2+ ATP-ase blocker that inhibits Ca2+ sparks, and iberiotoxin, a KCa channel blocker, reduced TNF-α-induced vasodilations to between 15 and 33% of control. In summary, data indicate that TNF-α activates NAD(P)H oxidase, resulting in an increase in intracellular H2O2 that stimulates Ca2+ sparks and transient KCa currents, leading to a reduction in global [Ca2+]i, and vasodilation.
doi:10.1152/ajpcell.00499.2005
PMCID: PMC1638900  PMID: 16267103
cerebrovascular circulation; ryanodine-sensitive calcium release channel; calcium-activated potassium channel; reactive oxygen species; vasodilation
17.  Modulation of the Local SR Ca2+ Release by Intracellular Mg2+ in Cardiac Myocytes 
The Journal of General Physiology  2008;132(6):721-730.
In cardiac muscle, Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) defines the amplitude and time course of the Ca2+ transient. The global elevation of the intracellular Ca2+ concentration arises from the spatial and temporal summation of elementary Ca2+ release events, Ca2+ sparks. Ca2+ sparks represent the concerted opening of a group of ryanodine receptors (RYRs), which are under the control of several modulatory proteins and diffusible cytoplasmic factors (e.g., Ca2+, Mg2+, and ATP). Here, we examined by which mechanism the free intracellular Mg2+ ([Mg2+]free) affects various Ca2+ spark parameters in permeabilized mouse ventricular myocytes, such as spark frequency, duration, rise time, and full width, at half magnitude and half maximal duration. Varying the levels of free ATP and Mg2+ in specifically designed solutions allowed us to separate the inhibition of RYRs by Mg2+ from the possible activation by ATP and Mg2+-ATP via the adenine binding site of the channel. Changes in [Mg2+]free generally led to biphasic alterations of the Ca2+ spark frequency. For example, lowering [Mg2+]free resulted in an abrupt increase of spark frequency, which slowly recovered toward the initial level, presumably as a result of SR Ca2+ depletion. Fitting the Ca2+ spark inhibition by [Mg2+]free with a Hill equation revealed a Ki of 0.1 mM. In conclusion, our results support the notion that local Ca2+ release and Ca2+ sparks are modulated by Mg2+ in the intracellular environment. This seems to occur predominantly by hindering Ca2+-dependent activation of the RYRs through competitive Mg2+ occupancy of the high-affinity activation site of the channels. These findings help to characterize CICR in cardiac muscle under normal and pathological conditions, where the levels of Mg2+ and ATP can change.
doi:10.1085/jgp.200810119
PMCID: PMC2585859  PMID: 19029377
18.  TNFα enhances microvascular tone and reduces blood flow in the cochlea via enhanced S1P signaling 
Objective
To demonstrate that TNFα, via sphingosine-1-phosphate (S1P) signaling, has the potential to alter cochlear blood flow and thus, cause ischemic hearing loss.
Methods and Results
TNFα induced a pro-constrictive state throughout the cochlear microvasculature, which reduced capillary diameter and cochlear blood flow in vivo. In vitro isolated preparations of the spiral modiolar artery and spiral ligament capillaries confirmed these observations. Antagonizing S1P receptor 2 subtype signaling (1µmol/L JTE013) attenuated the effects of TNFα in all models. TNFα activated Sk1 and induced its translocation to the smooth muscle cell membrane. Expression of a dominant-negative Sk1 mutant (Sk1G82D) eliminated both baseline spiral modiolar artery calcium sensitivity and TNFα effects, while a non-phosphorylatable Sk1 mutant (Sk1S225A) only blocked the effects of TNFα. A small group of etanercept-treated hearing loss patients recovered with a one-phase exponential decay (t½=1.56±0.20 weeks), which matched a kinetic predicted for a vascular origin.
Conclusions
TNFα indeed reduces cochlear blood flow via the activation of vascular S1P signaling. This integrates hearing loss into the family of ischemic microvascular pathologies, with implications for risk stratification, diagnosis and treatment.
doi:10.1161/STROKEAHA.110.593327
PMCID: PMC3404620  PMID: 20930159
Signal transduction; transfection; etanercept; sphingosine kinase 1; cochlear microcirculation
19.  Acidosis dilates brain parenchymal arterioles by conversion of calcium waves to sparks to activate BK channels 
Circulation research  2011;110(2):285-294.
Rationale
Acidosis is a powerful vasodilator signal in the brain circulation. However, the mechanisms by which this response occurs are not well understood, particularly in the cerebral microcirculation. One important mechanism to dilate cerebral (pial) arteries is by activation of large-conductance, calcium-sensitive potassium (BKCa) channels by local Ca2+ signals (Ca2+ sparks) through ryanodine receptors (RyRs). However, the role of this pathway in the brain microcirculation is not known.
Objective
The objectives of this study were to determine the mechanism by which acidosis dilates brain parenchymal arterioles (PAs) and to elucidate the roles of RyRs and BKCa channels in this response.
Methods and Results
Internal diameter and vascular smooth muscle cell (VSMC) Ca2+ signals were measured in isolated pressurized murine PAs, using imaging techniques. In physiological pH (7.4), VSMCs exhibited primarily RyR-dependent Ca2+ waves. Reducing external pH from 7.4 to 7.0 in both normocapnic and hypercapnic conditions decreased Ca2+ wave activity, and dramatically increased Ca2+ spark activity. Acidic pH caused a dilation of PAs which was inhibited by about 60% by BKCa channel or RyR blockers, in a non-additive manner. Similarly, dilator responses to acidosis were reduced by nearly 60% in arterioles from BKCa channel knockout mice. Dilations induced by acidic pH were unaltered by inhibitors of KATP channels or nitric oxide synthase.
Conclusions
These results support the novel concept that acidification, by converting Ca2+ waves to sparks, leads to the activation of BKCa channels to induce dilation of cerebral parenchymal arterioles.
doi:10.1161/CIRCRESAHA.111.258145
PMCID: PMC3505882  PMID: 22095728
brain parenchymal arteriole; acidosis; Ca2+ sparks; ryanodine receptor; potassium channel
20.  Calcium Sparks in Intact Skeletal Muscle Fibers of the Frog 
The Journal of General Physiology  2001;118(6):653-678.
Calcium sparks were studied in frog intact skeletal muscle fibers using a home-built confocal scanner whose point-spread function was estimated to be ∼0.21 μm in x and y and ∼0.51 μm in z. Observations were made at 17–20°C on fibers from Rana pipiens and Rana temporaria. Fibers were studied in two external solutions: normal Ringer's ([K+] = 2.5 mM; estimated membrane potential, −80 to −90 mV) and elevated [K+] Ringer's (most frequently, [K+] = 13 mM; estimated membrane potential, −60 to −65 mV). The frequency of sparks was 0.04–0.05 sarcomere−1 s−1 in normal Ringer's; the frequency increased approximately tenfold in 13 mM [K+] Ringer's. Spark properties in each solution were similar for the two species; they were also similar when scanned in the x and the y directions. From fits of standard functional forms to the temporal and spatial profiles of the sparks, the following mean values were estimated for the morphological parameters: rise time, ∼4 ms; peak amplitude, ∼1 ΔF/F (change in fluorescence divided by resting fluorescence); decay time constant, ∼5 ms; full duration at half maximum (FDHM), ∼6 ms; late offset, ∼0.01 ΔF/F; full width at half maximum (FWHM), ∼1.0 μm; mass (calculated as amplitude × 1.206 × FWHM3), 1.3–1.9 μm3. Although the rise time is similar to that measured previously in frog cut fibers (5–6 ms; 17–23°C), cut fiber sparks have a longer duration (FDHM, 9–15 ms), a wider extent (FWHM, 1.3–2.3 μm), and a strikingly larger mass (by 3–10-fold). Possible explanations for the increase in mass in cut fibers are a reduction in the Ca2+ buffering power of myoplasm in cut fibers and an increase in the flux of Ca2+ during release.
PMCID: PMC2229509  PMID: 11723160
ryanodine receptors; fluo-3; confocal microscopy; excitation-contraction coupling; frog muscle
21.  Identification and spatiotemporal characterisation of spontaneous Ca2+ sparks and global Ca2+ oscillations in retinal arteriolar smooth muscle cells 
Purpose
To identify spontaneous Ca2+ sparks and global Ca2+ oscillations in microvascular smooth muscle cells (MVSM) within intact retinal arterioles and to characterize their spatiotemporal properties and physiological functions.
Methods
Retinal arterioles were mechanically dispersed from freshly isolated rat retinae and loaded with the Ca2+-sensitive dye Fluo-4. Changes in [Ca2+]i were imaged in MVSM cells in situ using confocal scanning laser microscopy in XY or line scan mode.
Results
XY scans revealed both discretely localised, spontaneous Ca2+ events resembling Ca2+ sparks, and more global and prolonged Ca2+ transients which sometimes led to cell contraction. In line-scans, Ca2+ sparks were similar to those previously described in other types of smooth muscle with an amplitude (ΔF/F0) of 0.81±0.04 (mean±SE), Full Duration Half Maximum (FDHM) of 23.62±1.15 ms, Full Width Half Maximum (FWHM) of 1.25±0.05μm and frequency of 0.56±0.06 s−1. Approximately 35% of sparks had a prolonged tail (>80ms), similar to Ca2+ ‘embers’ described in skeletal muscle. Sparks often summated to generate global and prolonged Ca2+ elevations on which Ca2+ sparks were superimposed. These sparks occurred more frequently (2.86±025 s−1) and spread further across the cell (FWHM=1.67±0.08μm), but were smaller (ΔF/F0 = 0.69±0.04).
Conclusions
Retinal arterioles generate Ca2+ sparks whose characteristics vary during different phases of the spontaneous Ca2+-signalling cycle. Sparks summate to produce sustained Ca2+-transients associated with contraction and thus may play an important excitatory role in initiating vessel constriction. This deserves further study, not least because Ca2+ sparks appear to inhibit contraction in many other smooth muscle cells.
doi:10.1167/iovs.04-0719
PMCID: PMC2590679  PMID: 15557449
22.  RYR2 Proteins Contribute to the Formation of Ca2+ Sparks in Smooth Muscle 
The Journal of General Physiology  2004;123(4):377-386.
Calcium release through ryanodine receptors (RYR) activates calcium-dependent membrane conductances and plays an important role in excitation-contraction coupling in smooth muscle. The specific RYR isoforms associated with this release in smooth muscle, and the role of RYR-associated proteins such as FK506 binding proteins (FKBPs), has not been clearly established, however. FKBP12.6 proteins interact with RYR2 Ca2+ release channels and the absence of these proteins predictably alters the amplitude and kinetics of RYR2 unitary Ca2+ release events (Ca2+ sparks). To evaluate the role of specific RYR2 and FBKP12.6 proteins in Ca2+ release processes in smooth muscle, we compared spontaneous transient outward currents (STOCs), Ca2+ sparks, Ca2+-induced Ca2+ release, and Ca2+ waves in smooth muscle cells freshly isolated from wild-type, FKBP12.6−/−, and RYR3−/− mouse bladders. Consistent with a role of FKBP12.6 and RYR2 proteins in spontaneous Ca2+ sparks, we show that the frequency, amplitude, and kinetics of spontaneous, transient outward currents (STOCs) and spontaneous Ca2+ sparks are altered in FKBP12.6 deficient myocytes relative to wild-type and RYR3 null cells, which were not significantly different from each other. Ca2+ -induced Ca2+ release was similarly augmented in FKBP12.6−/−, but not in RYR3 null cells relative to wild-type. Finally, Ca2+ wave speed evoked by CICR was not different in RYR3 cells relative to control, indicating that these proteins are not necessary for normal Ca2+ wave propagation. The effect of FKBP12.6 deletion on the frequency, amplitude, and kinetics of spontaneous and evoked Ca2+ sparks in smooth muscle, and the finding of normal Ca2+ sparks and CICR in RYR3 null mice, indicate that Ca2+ release through RYR2 molecules contributes to the formation of spontaneous and evoked Ca2+ sparks, and associated STOCs, in smooth muscle.
doi:10.1085/jgp.200308999
PMCID: PMC2217466  PMID: 15024040
Ca2+ -induced Ca2+ release; ryanodine receptor; FKBP12.6; RYR3; knockout mouse
23.  Modulation of the Frequency of Spontaneous Sarcoplasmic Reticulum Ca2+ Release Events (Ca2+ Sparks) by Myoplasmic [Mg2+] in Frog Skeletal Muscle  
The Journal of General Physiology  1998;111(2):207-224.
The modulation by internal free [Mg2+] of spontaneous calcium release events (Ca2+ “sparks”) from the sarcoplasmic reticulum (SR) was studied in depolarized notched frog skeletal muscle fibers using a laser scanning confocal microscope in line-scan mode (x vs. t). Over the range of [Mg2+] from 0.13 to 1.86 mM, decreasing the [Mg2+] induced an increase in the frequency of calcium release events in proportion to [Mg2+]−1.6. The change of event frequency was not due to changes in [Mg-ATP] or [ATP]. Analysis of individual SR calcium release event properties showed that the variation in event frequency induced by the change of [Mg2+] was not accompanied by any changes in the spatiotemporal spread (i.e., spatial half width or temporal half duration) of Ca2+ sparks. The increase in event frequency also had no effect on the distribution of event amplitudes. Finally, the rise time of calcium sparks was independent of the [Mg2+], indicating that the open time of the SR channel or channels underlying spontaneous calcium release events was not altered by [Mg2+] over the range tested. These results suggest that in resting skeletal fibers, [Mg2+] modulates the SR calcium release channel opening frequency by modifying the average closed time of the channel without altering the open time. A kinetic reaction scheme consistent with our results and those of bilayer and SR vesicle experiments indicates that physiological levels of resting Mg2+ may inhibit channel opening by occupying the site for calcium activation of the SR calcium release channel.
PMCID: PMC2222774  PMID: 9450940
excitation–contraction coupling; Ca sparks; ryanodine receptor; magnesium; confocal microscopy
24.  Axial Stretch of Rat Single Ventricular Cardiomyocytes Causes an Acute and Transient Increase in Ca2+ Spark Rate 
Circulation research  2009;104(6):787-795.
We investigate acute effects of axial stretch, applied by carbon fibers (CFs), on diastolic Ca2± spark rate in rat isolated cardiomyocytes. CFs were attached either to both cell ends (to maximize the stretched region), or to the center and one end of the cell (to compare responses in stretched and nonstretched half-cells). Sarcomere length was increased by 8.01 ± 0.94% in the stretched cell fraction, and time series of XY confocal images were recorded to monitor diastolic Ca2± spark frequency and dynamics. Whole-cell stretch causes an acute increase of Ca2± spark rate (to 130.7 ± 6.4%) within 5 seconds, followed by a return to near background levels (to 104.4±5.1%) within 1 minute of sustained distension. Spark rate increased only in the stretched cell region, without significant differences in spark amplitude, time to peak, and decay time constants of sparks in stretched and nonstretched areas. Block of stretch-activated ion channels (2 gmol/L GsMTx-4), perfusion with Na±/Ca2±-free solution, and block of nitric oxide synthesis (1 mmol/L L-NAME) all had no effect on the stretch-induced acute increase in Ca2± spark rate. Conversely, interference with cytoskeletal integrity (2 hours of 10 gmol/L colchicine) abolished the response. Subsequent electron microscopic tomography confirmed the close approximation of microtubules with the T-tubular–sarcoplasmic reticulum complex (to within · 10−8m). In conclusion, axial stretch of rat cardiomyocytes acutely and transiently increases sarcoplasmic reticulum Ca2± spark rate via a mechanism that is independent of sarcolemmal stretch-activated ion channels, nitric oxide synthesis, or availability of extracellular calcium but that requires cytoskeletal integrity. The potential of microtubule-mediated modulation of ryanodine receptor function warrants further investigation.
doi:10.1161/CIRCRESAHA.108.193334
PMCID: PMC3522525  PMID: 19197074
mechanoelectric feedback; ryanodine receptor; stretch-activated channel; nitric oxide; electron microscopic tomography
25.  Hypoxia reduces KCa channel activity by inducing Ca2+ spark uncoupling in cerebral artery smooth muscle cells 
Arterial smooth muscle cell large-conductance Ca2+-activated potassium (KCa) channels have been implicated in modulating hypoxic dilation of systemic arteries, although this is controversial. KCa channel activity in arterial smooth muscle cells is controlled by localized intracellular Ca2+ transients, termed Ca2+ sparks, but hypoxic regulation of Ca2+ sparks and KCa channel activation by Ca2+ sparks has not been investigated. We report here that in voltage-clamped (−40 mV) cerebral artery smooth muscle cells, a reduction in dissolved O2 partial pressure from 150 to 15 mmHg reversibly decreased Ca2+ spark-induced transient KCa current frequency and amplitude to 61% and 76% of control, respectively. In contrast, hypoxia did not alter Ca2+ spark frequency, amplitude, global intracellular Ca2+ concentration, or sarcoplasmic reticulum Ca2+ load. Hypoxia reduced transient KCa current frequency by decreasing the percentage of Ca2+ sparks that activated a transient KCa current from 89% to 63%. Hypoxia reduced transient KCa current amplitude by attenuating the amplitude relationship between Ca2+ sparks that remained coupled and the evoked transient KCa currents. Consistent with these data, in inside-out patches at −40 mV hypoxia reduced KCa channel apparent Ca2+ sensitivity and increased the Kd for Ca2+ from ∼17 to 32 μM, but did not alter single-channel amplitude. In summary, data indicate that hypoxia reduces KCa channel apparent Ca2+ sensitivity via a mechanism that is independent of cytosolic signaling messengers, and this leads to uncoupling of KCa channels from Ca2+ sparks. Transient KCa current inhibition due to uncoupling would oppose hypoxic cerebrovascular dilation.
doi:10.1152/ajpcell.00629.2006
PMCID: PMC2241735  PMID: 17314264
transient; calcium-activated; potassium; current

Results 1-25 (1206078)