Smooth Muscle-selective Alternatively Spliced Exon Generates Functional Variation in Cav1.2 Calcium Channels

Voltage-gated calcium channels play a major role in many important processes including muscle contraction, neurotransmission, excitation-transcription coupling, and hormone secretion. To date, 10 calcium channel alpha(1)-subunits have been reported, of which four code for L-type calcium channels. In our previous work, we uncovered by transcript-scanning the presence of 19 alternatively spliced exons in the L-type Ca(v)1.2 alpha(1)-subunit. Here, we report the smooth muscle-selective expression of alternatively spliced exon 9(*) in Ca(v)1.2 channels found on arterial smooth muscle. Specific polyclonal antibody against exon 9(*) localized the intense expression of 9(*)-containing Ca(v)1.2 channels on the smooth muscle wall of arteries, but the expression on cardiac muscle was low. Whole-cell patch clamp recordings of the 9(*)-containing Ca(v)1.2 channels in HEK293 cells demonstrated -9 and -11-mV hyperpolarized shift in voltage-dependent activation and current-voltage relationships, respectively. The steady-state inactivation property and sensitivity to blockade by nifedipine of the +/-exon 9(*) splice variants were, however, not significantly different. Such cell-selective expression of an alternatively spliced exon strongly indicates the customization and fine tuning of calcium channel functions through alternative splicing of the pore-forming alpha(1)-subunit. The generation of proteomic variations by alternative splicing of the calcium channel Ca(v)1.2 alpha(1)-subunit can potentially provide a flexible mechanism for muscle or neuronal cells to respond to various physiological signals or to diseases.