Novel function of α1D L-type calcium channel in the atria

Abstract

Ca entry through atrial L-type Calcium channels (α_1C and α_1D) play an important role in muscular contraction, regulation of gene expression, and release of hormones including atrial natriuretic peptide (ANP), and brain natriuretic peptide (BNP). α_1D Ca channel is exclusively expressed in atria, and has been shown to play a key role in the pathogenesis of atrial fibrillation. Recent data have shown that the small conductance calcium-activated potassium channel, SK4 is also atrial specific and also contributes prominently to the secretion of ANP and BNP. However, its functional role in the heart is still poorly understood. Here we used α_1D gene heterozygous (α_1D^+/-) mice and HL-1 cells to determine the functional contribution of SK4 channels to α_1D-dependent regulation of ANP and BNP secretion in response to endothelin (ET), and/or mechanical stretch. Immunoprecipitation with α_1D specific antibody and western blotting with SK4 specific antibody on the immuno-precipitated protein complex showed a band at 50 KDa confirming the presence of SK4 in the complex and provided evidence of interaction between SK4 and α_1D channels. Using RT-PCR, we observed a 2.9 fold decrease in expression of Cacna1d (gene encoding α_1D) mRNA in atria from α_1D^+/-mice. The decrease in α_1D mRNA corresponded with a 4.2 fold decrease in Kcnn4 (gene encoding SK4) mRNA from α_1D^+/- mice. These changes were paralleled with a 77% decrease in BNP serum levels from α_1D^+/- mice. When α_1D was knocked down in HL-1cardiomyocytes using CRISPR/Cas9 technology, a 97% decrease in secreted BNP was observed even in cells subjected to stretch and endothelin. In conclusion, our data are first to show that α_1D Ca and SK4 channels are coupled in the atria, and that deletion of α_1D leads to decreased SK4 mRNA and BNP secretion providing evidence for a novel role of α_1D in atrial endocrine function. Elucidating the regulatory factors that underlie the secretory function of atria will identify novel therapeutic targets for treatment and prevention of cardiac arrhythmias such as atrial fibrillation.

Keywords: Alpha(1D) calcium channel; Atria; Atrial fibrillation; Brain natriuretic peptide.