A smooth muscle Cav1.2 calcium channel splice variant underlies hyperpolarized window current and enhanced state-dependent inhibition by nifedipine

Native smooth muscle L-type Ca(v)1.2 calcium channels have been shown to support a fraction of Ca2+ currents with a window current that is close to resting potential. The smooth muscle L-type Ca2+ channels are also more susceptible to inhibition by dihydropyridines (DHPs) than the cardiac channels. It was hypothesized that smooth muscle Cav1.2 channels exhibiting hyperpolarized shift in steady-state inactivation would contribute to larger inhibition by DHP, in addition to structural differences of the channels generated by alternative splicing that modulate DHP sensitivities. In addition, it has also been shown that alternative splicing modulates DHP sensitivities by generating structural differences in the Cav1.2 channels. Here, we report a smooth muscle L-type Cav1.2 calcium channel splice variant, Ca(v)1.2SM (1/8/9*/32/Delta 33), that when expressed in HEK 293 cells display hyperpolarized shifts for steady-state inactivation and activation potentials when compared with the established Ca(v)1.2b clone ( 1/8/9*/32/Delta 33). This variant activates from more negative potentials and generates a window current closer to resting membrane potential. We also identified the predominant cardiac isoform Ca(v)1.2CM clone ( 1a/8a/Delta 9*/32/33) that is different from the established Ca(v)1.2a (1a/8a/Delta 9*/31/33). Importantly, Cav1.2SM channels were shown to be more sensitive to nifedipine blockade than Cav1.2b and cardiac Cav1.2CM channels when currents were recorded in either 5 mM Ba2+ or 1.8 mM Ca2+ external solutions. This is the first time that a smooth muscle Cav1.2 splice variant has been identified functionally to possess biophysical property that can be linked to enhanced state-dependent block by DHP.