The SMA, a coiled artery inside the cochlea, is a branch of the anterior inferior cerebellar artery, which in turn branches off from the basilar artery located on the surface of the brain stem [1
]. The SMA is a small-caliber vessel (diameter ~ 60 μm) and has a single layer of smooth muscle cells that lacks tightly attached connective tissue [14
]. This architecture makes the SMA exquisitely suitable for investigating smooth muscle cell calcium regulation in small vessels (< 70 μm) using intact arteries, as opposed to isolated cells, which are devoid of their natural milieu. The major findings of this study using an intact vessel preparation of the SMA are as follows: (1) The SMA contains ryanodine receptors (RyRs) in the smooth muscle cells. (2) Smooth muscle cells of the SMA exhibit Ca2+
sparks, which are inhibited by ryanodine, an inhibitor of RyR. (3) Elevation of K+
and caffeine increased the frequency of Ca2+
sparks. The kinetics (rise time, decay time and amplitude) were not affected by K+
but were altered by caffeine. These results suggest that RyR-mediated Ca2+
sparks regulate smooth muscle contractility and play a role in the regulation of cochlear blood flow.
sparks, along with BK channel and Ca2+
channel currents play a prominent role in regulating myogenic tone of extra-cerebral arteries on the surface of the brain [5
sparks have been identified in many smooth muscle cells including smooth muscle cells from arteries [8
], portal vein [17
], urinary bladder [11
], airways [19
] and retinal arterioles [21
]. Interestingly, ryanodine-dependent Ca2+
sparks were detected in smooth muscle cells of cremaster feed arteries but not in cremaster arterioles [22
]. In this context, the observation of robust ryanodine-sensitive Ca2+
sparks in the smooth muscle cells of the intact SMA, a small-caliber third order branch of the basilar artery, is an important observation (Figure ).
The gerbil SMA was found to generate well-defined sparks at robust frequencies. Ca2+
sparks occurred at individual spark sites with an average frequency of 2.6 ± 0.1 Hz (Figure ). This frequency is higher than what has been reported for isolated smooth muscle cells [8
] as well as intact cerebral arteries [13
] and pressurized mesenteric arteries [16
]. Rise time was ~ 17 ms, which is roughly similar to the rise time of 20 ms reported for rat cerebral vessels [8
]. Time to half decay was ~ 21 ms, which is slightly faster than the 27 ms reported for rat portal vein [17
] and much faster than 48 - 65 ms reported in cerebral vessels [8
]. In fact, time to half decay of Ca2+
sparks in the gerbil SMA was more similar to the 23 - 37 ms reported for rat heart [26
]. Reasons for these differences may be due to species differences, due to individuality among blood vessels and differences in experimental conditions.
sparks are regulated by intracellular Ca2+
]. In smooth muscle cells, intracellular Ca2+
is strictly regulated by membrane potential. Membrane depolarization by electrical or chemical means or graded increases in intraluminal pressure increases intracellular Ca2+
influx and Ca2+
]. Membrane depolarization induced by elevation of K+
has been previously shown to induce vasoconstriction of the SMA, which is sensitive to block of L-type VDCCs [30
]. The observation that elevation of K+
spark frequency (Figure ) suggests that Ca2+
sparks play a role in the regulation of smooth muscle excitability and contractility.
In summary, the data presented here establish the intact spiral modiolar artery as an excellent model for the study of RyR-mediated Ca2+ signaling in smooth muscle cells of small arteries (< 70 μm) since it generates high amplitude Ca2+ sparks at robust frequencies. The data suggest a role for Ca2+ sparks in regulating vascular tone of the SMA and cochlear blood flow.