PMC

Mean FEV₁ % predicted by genotype and gender among combined SAGE and CHIRAH subjects (A) C818T (B) A1273G. Due to low numbers of 818TT homozygotes these subjects were combined with the CT heterozygotes. Error bars represent SEM. P-values listed are for the students t-test (C818T) and one-way ANOVA test (A1273G).

Structure of the human KCNMB1 gene with African American SNPs and their characteristics overlaid. SNPs are identified by mRNA NCBI no. NM_004137 position. Higher frequency allele for each SNP is listed first and minor allele last. Allele frequency among African Americans is listed. Amino acid changes are listed for coding SNPs. SNPs displayed in red were selected for genotyping in asthmatics cohorts.

At 1.7 but not 0.3 µm Ca²⁺, the Arg140Trp polymorphism decreases channel open probability over physiologically relevant voltage ranges. (A) Representative single-channel BK currents recorded at 1.7 µm Ca²⁺ from patches containing ∼25 α + β1_140Arg (left panel) and ∼64 α + β1_140Trp (right panels) channels. Traces were low-pass filtered at 4 kHz. (B) Single-channel current amplitude plotted as a function of voltage. (C) Averaged Log P_o–V relations at 1.7 mm [Ca²⁺]. Asterisks represent statistically significant difference P < 0.05 in unpaired student t-test. (D) Averaged Log P_o–V relations at 0.3 mm [Ca²⁺]. Symbols represent mean data for α + β1_140Arg (black symbol) and α + β1_140Trp (red symbol) and error bars represent SEM. In (B), the error bars are smaller than the symbols.

α + β1_140Trp channels have lower steady-state opening than α + β1_140Arg channels. Example current traces recorded from patches containing BK channels composed of α alone (A), α + β1_140Arg (C) and α + β1_140Trp (E). Mean G–V relations for α alone (B), α + β1_140Arg (D) and α + β1_140Trp (F) in 1.7μM Ca²⁺. Error bars represent SEM. Solid curves represent fits to a Boltzmann function (see Materials and Methods). Co-expression of β1_140Arg enhances channel opening by shifting the G–V relationship by ∼50 mV to negative potentials [(D) versus (B)]. β1_140Trp causes a delayed rise in current in the first 40 ms (E) and a smaller negative shift (35 mV) of the G–V curve (F). Gray trace in panel (F) is α + β1_140Arg (from D) for comparison.

Proposed sex-specific effects of C818T polymorphisms on airway smooth muscle (ASM) contractility. (A) BK channel α + β1-subunits are activated by local calcium from the sarcoplasmic reticulum, SR, (calcium-induced calcium release from ryanodine receptors). The β1-subunit is required for BK channel activation in response to local calcium (,,). The β1_140Arg (818C allele) confers a greater activation of BK channels than the β1_140Trp (818T allele). Complementing activation of BK channels is an estrogen-signaling cascade that activates nitric oxide (NO), guanylate cyclase (GC) and protein kinase G (PKG)-dependent phosphorylation of BK channels (,). (B) In 818C individuals, the β1_140Arg activates BK channels sufficiently to moderate ASM constriction in the absence of estrogen signaling. In African American females carrying the 818T allele, estrogen signaling sufficiently complements defects in BK channel activation caused by β1_140Trp. In African American 818T males, β1_140Trp reduced BK channel activation combined with the lack of estrogen signaling significantly enhancing airway constriction.