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Functional Characterization of the L-type Ca²⁺ Channel Ca_v1.41 from Mouse Retina

¹From the Department Pharmazie, Pharmakologie für Naturwissenschaften, Ludwig-Maximilians Universität Munich, Munich, Germany.

PURPOSE. To study the electrophysiological and pharmacological properties of the L-type Ca²⁺ channel (LTCC) Ca_v1.41 (1F) subunit from mouse retina and assess their contributions to the native retinal channel.

METHODS. The full-length cDNA of Ca_v1.41 was cloned from murine retina in an RT-PCR approach. Ca_v1.41 was expressed alone or together with the auxiliary 21 and ß2a or ß3 subunits in HEK293 cells. The electrophysiological and pharmacological characteristics of L-type Ca²⁺ and Ba²⁺ inward currents (I_Ca and I_Ba) induced by Ca_v1.41 were determined by the whole-cell configuration of the patch-clamp method and compared with currents induced by the cardiac and smooth muscle-type Ca_v1.21 (1C) channel.

RESULTS. Ca_v1.41-mediated I_Ba was observed only when the 21 and ß subunits were coexpressed. Current densities were approximately two times higher with ß2a than with ß3. I_Ba activated faster and revealed much slower time-dependent inactivation than I_Ba induced by Ca_v1.21. Unlike in Ca_v1.21, inactivation was not accelerated with Ca²⁺ as the charge carrier, indicating the absence of Ca²⁺-dependent inactivation in Ca_v1.41. Ca_v1.41 exhibited voltage-dependent inactivation. The dihydropyridine (DHP) antagonist isradipine blocked Ca_v1.41 with approximately 20-fold lower sensitivity than Ca_v1.21. The agonistic DHP BayK 8644 stimulated maximum I_Ba approximately sixfold. Ca_v1.41 revealed only moderate sensitivities to L- and D-cis-diltiazem, with IC₅₀ in the micromolar range. Both enantiomers unexpectedly blocked Ca_v1.41 with almost equal IC₅₀.

CONCLUSIONS. The data indicate that Ca_v1.41 subunit constitutes the major molecular correlate of retinal L-type Ca²⁺ current. Its intrinsic biophysical properties, in particular its unique inactivation properties, enable Ca_v1.41 to provide a sustained I_Ca over a voltage range such as required for tonic glutamate release at the photoreceptor synapse.

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