Functional Characterization of the L-type Ca Channel Cav1.4 1 from Mouse Retina

METHODS. The full-length cDNA of Cav1.4 1 was cloned from murine retina in an RT-PCR approach. Cav1.4 1 was expressed alone or together with the auxiliary 2 1 and 2a or 3 subunits in HEK293 cells. The electrophysiological and pharmacological characteristics of L-type Ca and Ba inward currents (ICa and IBa) induced by Cav1.4 1 were determined by the whole-cell configuration of the patch-clamp method and compared with currents induced by the cardiac and smooth muscle-type Cav1.2 1 ( 1C) channel. RESULTS. Cav1.4 1-mediated IBa was observed only when the 2 1 and subunits were coexpressed. Current densities were approximately two times higher with 2a than with 3. IBa activated faster and revealed much slower time-dependent inactivation than IBa induced by Cav1.2 1. Unlike in Cav1.2 1, inactivation was not accelerated with Ca as the charge carrier, indicating the absence of Ca -dependent inactivation in Cav1.4 1. Cav1.4 1 exhibited voltage-dependent inactivation. The dihydropyridine (DHP) antagonist isradipine blocked Cav1.4 1 with approximately 20-fold lower sensitivity than Cav1.2 1. The agonistic DHP BayK 8644 stimulated maximum IBa approximately sixfold. Cav1.4 1 revealed only moderate sensitivities to Land D-cis-diltiazem, with IC50 in the micromolar range. Both enantiomers unexpectedly blocked Cav1.4 1 with almost equal IC50. CONCLUSIONS. The data indicate that Cav1.4 1 subunit constitutes the major molecular correlate of retinal L-type Ca current. Its intrinsic biophysical properties, in particular its unique inactivation properties, enable Cav1.4 1 to provide a sustained ICa over a voltage range such as required for tonic glutamate release at the photoreceptor synapse. (Invest Ophthalmol Vis Sci. 2004;45:708–713) DOI:10.1167/iovs.03-0937