alpha 1D (Cav1.3) subunits can form L-type Ca2+ channels activating at negative voltages

In cochlea inner hair cells (IHCs), L-type Ca2+ channels (LTCCs) formed by alpha 1D subunits (D-LTCCs) possess biophysical and pharmacological propert ies distinct from those of alpha 1C containing C-LTCCs. We investigated to which extent these differences are determined by alpha 1D itself by analyzi ng the biophysical and pharmacological properties of cloned human alpha 1D splice variants in tsA-201 cells. Variant alpha 1Ds(8A) containing exon 8A sequence in repeat I, yielded alpha 1D protein and L-type currents, whereas no intact protein and currents were observed after expression with exon 8B . In whole cell patch-clamp recordings (charge carrier 15-20 mM Ba2+), alph a 1D(8A) - mediated currents activated at more negative voltages (activatio n threshold, -45.7 versus -31.5 mV, p < 0.05) and more rapidly (tau (act) f or maximal inward currents 0.8 versus 2.3 ms; p < 0.05) than currents media ted by rabbit cylC, Inactivation during depolarizing pulses was slower than for alpha 1C (current inactivation after 5-s depolarizations by 90 versus 99%, p < 0.05) but faster than for LTCCs in IHCs. The sensitivity for the d ihydropyridine (DHP) L type channel blocker isradipine was 8.5-fold lower t han for alpha 1C. Radioligand binding experiments revealed that this was no t due to a lower affinity for the DHP binding pocket, suggesting that diffe rences in the voltage dependence of DHP block account for decreased sensiti vity of D-LTCCs, Our experiments show that alpha 1D(8A) subunits can form s lowly inactivating LTCCs activating at more negative voltages than alpha 1C , These properties should allow D-LTCCs to control physiological processes, such as diastolic depolarization in sinoatrial node cells, neurotransmitte r release in IHCs and neuronal excitability.