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A highly heritable and reproducible measure of asthma severity is baseline pulmonary function. Pulmonary function is largely determined by airway smooth muscle (ASM) tone and contractility. The large conductance, voltage and calcium-activated potassium (BK) channel negatively regulates smooth muscle tone and contraction in ASM. The modulatory subunit of BK channels, the β1-subunit, is critical for proper activation of BK channels in smooth muscle and has shown sex hormone specific regulation. We hypothesized that KCNMB1 genetic variants in African Americans may underlie differences in bronchial smooth muscle tone and thus pulmonary function, possibly in a sex-specific manner. Through resequencing of the KCNMB1 gene we identified several common variants including a novel African-specific coding polymorphism (C818T, R140W). The C818T SNP and four other KCNMB1 variants were genotyped in two independent groups of African American asthmatics (n = 509) and tested for association with the pulmonary function measure – forced expiratory volume (FEV1) % of predicted value. The 818T allele is associated with a clinically significant decline (−13%) in FEV1 in both cohorts of asthmatics among males but not females (Pcombined = 0.0003). Patch clamp electrophysiology studies of the BK channel expressed with the 140Trp variant of the β1-subunit demonstrated significantly reduced channel openings, predicted by the loss of pulmonary function observed. African American male asthmatics carrying the 818T allele (10% of population) are potentially at risk for greater airway obstruction and increased asthma morbidity. Female asthmatics may be insulated from the deleterious effects of the 818T allele by estrogen-mediated upregulation in BK channel activity.
Asthma is a complex disease characterized by persistent airway inflammation giving rise to bronchial hyperresponsiveness and reversible airway obstruction. The degree of airway obstruction is an important criterion for classifying disease severity (1,2). In fact, severe airway obstruction is a significant risk factor for near fatal asthma (3). Therefore, understanding factors contributing to differences in pulmonary function is important to the management of asthma morbidity and mortality. Airway obstruction varies greatly among subjects. Both family-based and twin studies have established a strong heritability of pulmonary function (4–7). Genetic studies have associated multiple loci and individual variants with airway obstruction or pulmonary function (8–11).
Airway smooth muscle (ASM) tone and thus pulmonary function is largely determined by cholinergic stimulation, which is mediated through muscarinic receptors (12). Muscarinic receptors stimulate contraction by increasing intracellular calcium concentrations. An important negative regulator of this process is the large conductance, Ca2+ and voltage-dependent K+ (BK) channel (13). BK channel potassium conductance is stimulated by an increase in local calcium concentration, thereby moderating depolarization, deactivating voltage-dependent calcium channels and thus reducing overall calcium influx and the resultant airway constriction (14). BK channels are formed by pore-forming α-subunits alone, or with modulatory β1-subunits. Sensitivity of the BK channel to Ca2+ influx is modulated by the β1-subunit (15). The β1-subunit is abundantly expressed in bronchial smooth muscle (14). The β1-subunit is encoded for by the KCNMB1 gene, which is located in the 5q34 chromosomal region, a locus that has been frequently linked with asthma and baseline lung function as denoted by forced expiratory volume (FEV1) (8,10,16,17). We hypothesized that genetic variation in the KCNMB1 gene would alter the function and/or expression of the β1-subunit, which could then modulate pulmonary function among subjects with asthma.
This hypothesis is supported by the phenotype observed from β1-subunit knockout mice which display reduced BK channel activity resulting in increased smooth muscle tone in the vasculature (15,18), bladder (19) and trachea (14). In fact, disruption of vascular tone in β1-knockout mice results in hypertension and left-ventricular hypertrophy (15,18). Based on the functional significance of the β1-subunit on vascular smooth muscle tone, genetic studies have been conducted on the KCNMB1 gene for association with hypertension and related cardiovascular phenotypes. A German twin study associated both intronic and coding variants with baroreflex and regulation of blood pressure (20). Most notably, a large study identified the E65K (a gain-of-function) coding variant in KCNMB1 as protective against high diastolic blood pressure in a Spanish cohort (21). Furthermore, in a separate study the protective effect of the E65K variant on blood pressure and cardiovascular events was determined to be largely dependent on sex and age, with post-menopausal women showing the strongest effect of the variant on hypertension (22). This result raises another important aspect of BK channels, in that their activity and expression is upregulated by female sex hormones including estrogen and estradiol (23,24). In fact, the protective effect of estrogen pretreatment on murine ASM against hyperresponsiveness, was determined to be primarily mediated by a 50-fold increase in BK channel activity (23,24).
Based on the existing KCNMB1 genetic results for hypertension coupled with the importance of the β1-subunit in regulation of ASM tone, we undertook a study to determine whether genetic variation in the KCNMB1 gene modified asthma severity among African Americans and whether there were differences by sex. We chose to study African American subjects with asthma because they have among the highest asthma morbidity and mortality rates in the US.
We identified all KCNMB1 exonic genetic variants and determined their corresponding allele frequencies by resequencing the four KCNMB1 exons in 24 African American subjects with asthma. We identified eight SNPs including two – 5′-untranslated region (UTR) SNPs, three – coding SNPs, and three – 3′-UTR SNPs (Fig. 1). Interestingly, the polymorphism associated with hypertension (G593A Glu65Lys), had a much lower allele frequency in African Americans (5.2%) than previously reported among Caucasians (17.5%) (25). Additionally, we identified two previously unreported SNPs (A274C and C818T). SNP C818T is a novel African-specific coding polymorphism resulting in an Arginine to Tryptophan change in the KCNMB1 protein at amino acid position 140. We did not find a single subject heterozygous for the C818T SNP in screening 96 Puerto Rican, 96 Mexican, 86 Caucasian, and seven Asians asthmatics, implying this SNP is specific to populations of African origin.
Pairwise linkage disequilibrium (LD) analysis of the identified SNPs by r2 statistic revealed limited correlation between the eight SNPs as shown in Table 1. The exceptions were the adjacent SNPs C1054A and A1055C, which were in complete LD among African Americans. Additionally, we found SNP G728C and A1273G to be moderately correlated with an r2 of 0.43.
Based upon allele frequencies and LD patterns determined in the resequencing analysis, we selected SNPs G593A, G728C, C818T, C1054A and A1273G for genotyping in 261 African Americans from the Study of African Americans, Asthma, Genes, and Environments (SAGE) (Supplementary Material, Tables S1 and S2) (Table 2). We first tested for association between the quantitative measure of pulmonary function, FEV1 % of predicted value, and the five KCNMB1 SNPs by multiple linear regression correcting for potential confounding factors (Table 3). We did not observe any significant associations in the combined sex analysis. Because of the known effects of female sex hormones on BK channels, we performed a sex-stratified analysis to determine if there was effect modification by sex on the risk associated with KCNMB1 variants. In the sex-stratified analysis, we observed significant associations between FEV1 % of predicted value and SNPs C818T (P = 0.018) and A1273G (P = 0.054), only among males subjects (Table 3). The regression coefficient for C818T was −13.73 (T allele confers a decrease in lung function), while the coefficient was positive 5.38 (an increase in lung function) for SNP A1273G (Table 3). No association with any of the five SNPs was observed among female subjects. To confirm our results with an independent group of African American subjects with asthma, we genotyped 248 subjects from the Chicago Initiative to Raise Asthma Health Equity (CHIRAH) study (Supplementary Material, Table S1). The same five KCNMB1 SNPs examined in the SAGE asthmatics were also genotyped in the CHIRAH asthmatics (Table 2). As in the SAGE analysis, the sex-stratified analysis in CHIRAH yielded significant results with SNP C818T (P = 0.003) and borderline significant results with SNP A1273G (P = 0.093) only among male subjects (Table 3). The direction of the association with both SNPs was replicated with similar coefficients, namely the 818T allele conferred lower pulmonary function (Coeff. = −13.22), and the 1273 G allele conferred greater lung function (Coeff. = 4.46) (Table 3). We did not observe any significant associations between KCNMB1 SNPs and FEV1 % of predicted value among female asthmatics (Table 3).
In an attempt to increase the power of our stratified analysis of FEV1 and refine our genetic coefficient estimates for replicated SNPs C818T and A1273G, we analyzed these SNPs in the combined group of 509 SAGE and CHIRAH asthmatics. Multiple linear regression of FEV1 % of predicted value yielded significant results among males for both SNP C818T (Coeff. = −13.1, P = 0.0003) and SNP A1273G (Coeff. = 4.96, P = 0.01) (Table 3). Based on these results we tested the interaction term between C818T and sex, which we found to be significant as well (P = 0.002). We performed many analyses, which are important on their own; however it is important to assess the burden of multiple comparisons. We performed 3 × 5 × 3 = 45 tests in all (three for sex: males, females, both sexes; five SNPs; three for cohorts: SAGE, CHIRAH, both cohorts). Disregarding the correlations between the tests, applying the Bonferroni correction, the adjusted P-value of C818T association with FEV1 is still significant (45 × 0.0003 = 0.0135). Moreover, male asthmatics with the CC genotype for SNP C818T have a mean FEV1 % predicted of 92.0%, whereas asthmatics in the CT and TT genotype group (combined due to low numbers of TT homozygotes) have a mean FEV1 % predicted of 80.2% reflecting the regression results and implying a dominant effect of the T allele (P = 0.002) (Fig. 2A). Among male subjects, the mean FEV1 % for genotypes AA, AG and GG of the SNP + A1273G were 88.1, 92.9 and 101.9%, respectively. These results are consistent with an additive effect of SNP A1273G (P = 0.022) (Fig. 2B).
Based upon these results, we chose to explore the effect of these SNPs on other relevant measures of airway obstruction including, forced expiratory flow in 25–75 s (FEF25–75), Forced Vital Capacity (FVC%) and FEV1/FVC ratio. To avoid multiple testing problems and to take advantage of the correlated nature of these lung function measures we used principal components analysis (PCA) to create a new variable representing pulmonary function in general. PCA yielded two components with Eigen values >1. The first component accounted for 67% of the total variance in these pulmonary function measures (Table 4). Linear regression was used to test for association of the first principal component and SNPs C818T and A1273G in the combined group of asthmatics (SAGE and CHIRAH). We found no association with either SNP among all asthmatics or among females. However, similar to the genotype analysis for FEV1, both C818T and A1273G SNPs were associated with the first principal component of pulmonary function (P = 0.001 negative coefficient and P = 0.011 positive coefficient, respectively) only among male subjects. These results strongly support the involvement of KCNMB1 genetic variants in male subjects on multiple measures of airway obstruction.
The C818T allele results in a change from Arginine to Tryptophan at amino acid 140 of the KCNMB1 protein. Alignment of mammalian KCNMB1 sequences indicates that this polymorphism is not observed in other mammalian KCNMB1 sequences (data not shown). The non-conserved change from Arginine, a basic amino acid, to tryptophan, an amino acid with a bulky non-polar side chain, suggests that such a change may have effects on KCNMB1 function. To investigate the consequence of the C818T polymorphism on BK channel function, the human KCNMB1 140Arg (818C) or 140Trp (818T) cDNAs were coexpressed with the BK channel pore-forming α-subunit in HEK293 cells and studied using inside-out, excised patch clamp recording. Figure 3 shows representative current traces for α-subunit alone, α + β1140Arg and α + β1140Trp evoked by a series of voltage steps. α-Subunit currents alone have relatively fast activation and deactivation (Fig. 3A). Plots of normalized conductance demonstrate half-maximal openings (V1/2) at 79.9 ± 5.9 mV (Fig. 3B). As shown previously (26), the β1140Arg-subunit slows activation and deactivation of BK channels (Fig. 3C) and shifts the V1/2 by 50 mV to more negative voltages (Fig. 3D, V1/2 is 30.6 ± 2.7 mV). In contrast, β1140Trp-subunit confers a smaller G–V shift of 34 mV (Fig. 3F) resulting in BK channels that require a greater depolarization to open (average V1/2 is 46.0 ± 2.9 mV). Table 5 shows a plot of average V1/2 values measured across a range of calcium concentrations. The effect of the Trp140 residue is significant at intermediate calcium concentrations of 1.7 (shown in Fig. 3) and 7 µM where there is a 12–15 mV positive shift of the V1/2 relative to 140Arg containing channels. The apparent voltage dependence (Q) is not altered by the polymorphism (Table 5). A comparison of activation and deactivation kinetics of the KCNMB1 polymorphism demonstrates that the 140Trp polymorphism causes >2-fold slowing of activation time over 140Arg containing subunits (e.g. at +50 mV and 1.7 µM Ca2+: β1140Arg is 53 ± 14 ms versus β1140Trp 135 ± 34 ms, P = 0.02) (Supplementary Material, Fig. S1A) without affecting deactivation (Supplementary Material, Fig. S1B).
To understand the effect of the 140Trp polymorphism at physiological voltages, we measured channel open probability at voltages where ASM fluctuates (27). Figure 4A shows an example of single-channel recordings of α + β1140Arg and α + β1140Trp channels. We found that β1140Trp has a somewhat smaller single-channel current than β1140Arg at negative voltages (15.6 ± 0.8 pA for β1140Trp versus 17 ± 0.7 pA for β1140Arg at −80 mV, P < 0.005) (Fig. 4B). Consistent with the positive shift of the V1/2, the 140Trp polymorphism reduces the BK channel open probability (Po) relative to β1140Arg containing channels (Fig. 4C). For example, there is a 3.3-fold decrease in Po of α + β1140Trp versus β1140Arg (P = 0.027) at −80 mV and 1.7 µM Ca2+. Interestingly, there is no difference in open probability between β1140Arg and β1140Trp channels at 0.3 µm calcium (Fig. 4D). The calcium-dependent effect on open probability indicates that the 140Trp polymorphism reduces BK channel activity through an effect on the apparent calcium sensitivity. In summary, these results suggest that individuals carrying the 818T allele (140Trp polymorphism) will have BK channels with reduced openings which will likely result in membrane depolarization and therefore more constricted airways.
We have demonstrated significant associations between multiple exonic variants in the KCNMB1 gene and asthma severity as defined by baseline pulmonary function in two independent groups of African American asthmatics. Our results indicate a complex and important role of KCNMB1 genetic variation in pulmonary function, which is sexually dimorphic. Namely, we have observed a novel amino acid changing variant in the KCNMB1 gene, C818T R140W, which is strongly associated with a clinically significant decrease in FEV1 % of predicted value only among African American male subjects in two independent groups of asthmatics. In fact, subjects either heterozygous or homozygous for the 818T allele had a mean FEV1 of 80.2%, indicating moderate airway obstruction, compared with the mild airway obstruction exhibited by the 818 CC genotype group with a mean FEV1 of 92.0%. Furthermore, the KCNMB1 3′-UTR SNP, A1273G, was associated with increased FEV1 % of predicted value, also among male subjects. The additive increase in FEV1 by the A1273G variant was observed in both groups of asthmatic males. Additionally, we used PCA to create a summary variable (principal component) of pulmonary function representing FEV1, FVC, FEF25–75 and FEV1/FVC. Again we observed strong negative effects of the 818T allele on this general measure of pulmonary function, solidifying the effect of this SNP on African American males with asthma. Similar to C818T, SNP A1273G was also associated with the PCA generated measure of pulmonary function among males.
Our exploration of the potential effects of KCNMB1 genetic variants on pulmonary function was motivated by the vital role that BK channels play in the regulation of arterial and bronchial smooth muscle contraction and the subsequent studies relating genetic variants in the KCNMB1 gene with modulation of blood pressure. Our results are consistent with functional studies reporting the importance of BK channels in ASM contraction due to their high potassium conductance and abundant expression in ASM (14,23). The ASM of β1 knock-out mice shows increased resting calcium levels and calcium influx during contraction, revealing the importance of this subunit in moderating airway constriction (14).
Sexual dimorphism in asthma and related traits is well established. There are well-documented differences by sex with respect to lung development, age of asthma onset, asthma severity, and rate of pulmonary function decline (28–31). Sex modification of genotype–phenotype associations in asthma has precedence including, male-specific associations between vitamin D receptor variants and both asthma and IgE levels, male-specific association of β2-adrenergic receptor polymorphisms and persistence of asthma, female-specific associations of estrogen receptor polymorphisms with both airway hyperresponsiveness and rate of lung function decline (32–35). Moreover, a prior study of KCNMB1 G593A E65K variant indicated its effects were sex-specific and primarily present in post-menopausal women (22). We propose that these effects maybe hormonally mediated since BK channel activity is upregulated by female sex hormones including estrogen and estradiol (23) (Fig. 5A). The mechanism behind estrogen activation of BK channels is an acute non-genomic event triggered by a signaling cascade, which has been established in murine ASM cells (23). Namely, estrogen stimulates the production of NO from Nitric oxide synthases, which then activates guanylate cyclases/protein kinase G (PKG) (36). Subsequently PKG phosphorylates BK channels resulting in a dramatic 50-fold increase in BK channel activity (Fig. 5A) (23). Indeed in women low estrogen levels experienced during the premenstrual luteal phase have been associated with both asthma exacerbation and decline in pulmonary function, as opposed to ovulation, when estrogen levels are highest, and asthma severity is decreased (37–39). In our study the median age of females was 22.1 years with the oldest being 40 years, implying that all female participants are pre-menopausal. The upregulation of BK channel activity mediated by female sex hormones may insulate pre-menopausal women from the moderate functional effects of KCNMB1 variants, whereas these same moderate genetic effects may push males and post-menopausal women over a critical threshold resulting in disease, as observed in this study and in the study of hypertension (22) (Fig. 5B).
Despite the replication of our genetic association with pulmonary function and sex modification, we sought to characterize the effect of the C818T coding variant on BK channel function as further validation of the clinical effects observed in this study. We found the β1140Trp isoform (encoded by 818T allele) to confer a partial loss of function by significantly reducing open probability of the BK channel compared with the β1140Arg isoform (encoded by 818C allele). This is consistent with the loss in pulmonary function observed in carriers of the 818T, since BK channels act as a brake on muscarinic signaling-induced bronchial smooth muscle contraction and tone (14). These results give mechanistic and biologic plausibility to the genetic results observed for the C818T variant (Fig. 5). Based upon these results we hypothesize that the increase in lung function observed for 1273 G allele carriers may be mediated by an upregulation in expression and/or activity of the BK channel. We did not find an association with SNP G593A despite prior reports of channel gain-of-function conferred by this variant and association with hypertension (21). However, these results were obtained in populations of European origin where the G593A allele is present at a much higher frequency. The low allele frequency in African Americans coupled with the loss in power due to stratification by sex likely contributed to this lack of association. Further studies in a European population will be needed to confirm our result.
Interestingly, we found the associated 818T allele to be African-specific. In fact, we were unable to detect the 818T allele after screening large numbers of Caucasian, Asian, Puerto Rican, and Mexican subjects. This explains the undiscovered nature of this variant prior to our study despite multiple genetic studies of hypertension and KCNMB1. Our results underscore the need to examine racially and ethnically diverse populations in genetic research. These racial specific variants may explain in part, along with social, economic, and environmental variables the significant disparities in asthma burden observed among African Americans (40,41). Our results have important clinical and public health implications. In the US, African Americans have nearly the highest asthma morbidity and mortality rates with prior studies finding significantly lower FEV1 values among African American asthmatics compared with other ethnicities (42). Based on our results 10% of African American males with asthma carry at least one copy of the 818T allele, potentially putting them at higher risk for asthma morbidity and mortality. These findings bring us closer to understanding the pharmacogenetic profiles of different populations and create the possibility for more tailored drug design for asthma. Indeed substances that activate BK channels through the β1-subunit, such as dehydrosoyasaponin-1 (43) should be visited as potential asthma therapeutics, particularly for individuals carrying the 818T allele.
Furthermore, the effect of this variant maybe of additional significance in other disease states such as hypertension, which also disproportionately affects African American males (44). The other associated SNP, A1273G, is common among other ethnic groups including Caucasians (11%), Mexicans (15%) and Puerto Ricans (15%); therefore the effects mediated by this SNP may extend to other populations as well (25).
In conclusion, our results indicate that both the C818T and A1273G variants in the KCNMB1 gene strongly influence pulmonary function among African American males with asthma. We find these genetic effects to be restricted to male subjects, possibly due to hormonal differences. The effects of both the C818T and A1273G variants on baseline pulmonary function are clinically significant and as such are important indicators of asthma severity. Results of the functional studies of the C818T encoded isoform revealed a significant loss-of-function of the BK channel with the (T) allele. Further studies will be needed to establish the role of the A1273G variant in pulmonary function among other populations. Additionally, studies are warranted for bronchial hyperresponsiveness considering the established role of BK channel conductance in controlling cholinergic stimulated contraction of bronchial smooth muscle.
The resequencing analysis was performed on our SNP Discovery Panel, which contains 24 African American asthmatic subjects. Recruitment for the SAGE study is ongoing and includes 261 asthmatic cases from the San Francisco Bay Area (45,46). African American subjects from the CHIRAH were used as an independent replicate population. The CHIRAH cohort is a community-based longitudinal cohort study of urban children and adults with persistent asthma (47). Studies are described in detail in the online supplement with characteristics listed in Supplementary Material, Table S1. Local Institutional Review Boards approved all the studies, and all subjects provided written, age-appropriate informed consent or assent.
Pulmonary function was evaluated using spirometry performed according to American Thoracic Society standards (48). Subjects were instructed to abstain from all bronchodilator medications for 8 h prior to all pulmonary function tests. Spirometric reference values from Hankinson et al. (49) were used to determine percent predicted values.
All KCNMB1 exons (four) and exon–intron boundaries (20 bp from either end of the exon) were sequenced in the SNP Discovery panel. Sequencing details and primers are listed in the Supplementary Material, Table S2.
SNPs G593A, G728C, C818T, C1054A and A1273G were genotyped in the SAGE and CHIRAH cohorts. All genotyping methods and primers are described in the online supplement (Supplementary Material, Tables S3 and S4).
The χ2 goodness-of-fit test was used to determine whether the individual KCNMB1 SNPs were under Hardy Weinberg equilibrium (HWE). All SNPs genotyped conformed to HWE (P > 0.01) in both sets of asthmatic cases. LD patterns were determined by using the r2 statistic (Table 1). Multiple linear regression was used to test association between KCNMB1 SNP genotypes and the degree of airway obstruction as determined by FEV in 1 s percent of predicted value (FEV1 %) in the SAGE and CHIRAH asthma cases. Genotypes were coded in an additive fashion by default for SNPs with large minor allele frequencies (MAF) (C1054A and A1273G), and dominantly coded for SNPs with a small MAF (G593A, G728C, C818T) due to the small number of minor allele homozygotes. Age, sex, and individual ancestry estimates (IAE) were all included as covariates in SAGE and CHIRAH linear regression models. Additionally, we tested use of short-acting β2-agonists, inhaled corticosteroids, oral corticosteroids (not tested or available in CHIRAH), and asthma duration for their inclusion into the regression models due to their potential for confounding. The F-statistic was used to determine if these variables affected the regression models. The IAE variable was generated by genotyping 60 ancestry informative markers in all SAGE and CHIRAH subjects and analyzing this data along with ancestral genotypes in the program STRUCTURE as is described elsewhere (50,51). Sex-stratified analyses were repeated in the same fashion. In the combined analysis of both SAGE and CHIRAH subjects the regression models included age, sex, and IAE and an additional covariate for each specific study (SAGE or CHIRAH).
Measures of pulmonary function are highly correlated. Therefore, we used PCA using the correlation matrix between variables to create a single summary variable representing pulmonary function. The pulmonary function measures included in the PCA are FEV1, FVC, FEF25–75 and FEV1/FVC. PCA was used to create components of these four variables that together account for the total variance in these measures. Components with Eigen values >1.0 were kept and scores were created for these components with a varimax rotation method. These scores were then tested as a general measure of pulmonary function for association with SNPs C818T and A1273G by linear regression. Regression models were run as described earlier. All statistical analyses were performed using STATA 8.0 S/E statistical software (College Station, TX, USA).
Effects of β1140Arg and β1140Trp on BK channel gating were characterized in HEK293T cells. The human β1 cDNA (NM_004137) was mutated to introduce the C818T (R140W) polymorphism using the Stratagene Quickchange mutagenesis kit and confirmed by sequencing. The original human α-subunit cDNA (U11058) initiates translation at the third in-frame translation initiation site, methionine-aspartic acid-alanine-leucine (52). The construct was therefore appended at the 5′ end to include the first translation initiation site (methionine-alanine-asparagine) and downstream sequence that was absent from this expression construct. The human α-subunit, β1-subunit and EGFP-N1 plasmids were cotransfected at a ratio of 1:10:1 to ensure saturation of BK channels with β1-subunits and enable identification of transfected (green fluorescent protein fluorescent) cells.
Ionic current recordings were made using the inside-out patch clamp configuration according to previously described methods (26). Conductance–voltage (G–V) relationships were obtained using test pulses that were followed by a step to a tail voltage (+60 at 0.3 µM Ca2+, −80 at higher Ca2+), and then measuring instantaneous tail current 200 μs after the test pulse. Tail current amplitudes (G) were normalized to tail currents at maximal conductance (Gmax). In experiments where Gmax were not reached, Gmax values at higher Ca2+ from the same patch were used. G/Gmax–V data were fitted with the Boltzman function: , where V is the test potential, V1/2 is the membrane potential at half-maximal conductance, Q is the equivalent gating charge, and F, R and T are constants.
Single-channel opening events were obtained from patches containing tens to hundreds of channels. Recordings were of 20 s to hundreds of seconds in duration. Analysis was performed using TAC and TACFIT programs (Bruxton Corporations). NPo was determined using either all-point amplitude histogram or by event detection using a 50% amplitude criteria. The probability (Pk) of occupying each open level (k) gives rise to NPo: NPo = ∑kkPk. Po was then determined by normalizing NPo values by channel number (N). N was estimated by dividing the maximum instantaneous tail current amplitude by the unitary conductance.
This work was supported by National Institutes of Health [HL078885 (E.G.B.), HL078860 and HL068546 (R.S.)], HL082197 (B.W.), Flight Attendant Medical Research Institute (FAMRI) (E.G.B.), RWJ Amos Medical Faculty Development Award, NCMHD Health Disparities Scholar, Extramural Clinical Research Loan Repayment Program for Individuals from Disadvantaged Backgrounds, 2001–2003, (E.G.B.), Chicago Initiative to Raise Asthma Health Equity (CHIRAH) is funded by the National Heart Lung and Blood Institute (NHLBI), 5U01 HL072478-05, The Ernest S. Bazley Grant to Northwestern Memorial Hospital and Northwestern University and the Sandler Asthma Basic Research Center (SABRE), Sandler Program for Asthma Research (SPAR) and the Sandler Family Supporting Foundation. Support for genotyping was provided by the National Center for Minority Health Disparities Center of Excellence in Nutritional Genomics (P60 MD00022).
Conflict of Interest statement. None declared.