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(Circulation Research. 2001;89:700.)
© 2001 American Heart Association, Inc.

Cellular Biology

On the Role of Ca²⁺- and Voltage-Dependent Inactivation in Ca_v1.2 Sensitivity for the Phenylalkylamine (-)Gallopamil

Stanislav Sokolov, Eugen Timin, Steffen Hering

From the Institut für Biochemische Pharmakologie (S.S., S.H.), Innsbruck, Austria; and A.V. Vishnevsky Institute of Surgery (E.T.), Moscow, Russia.

Correspondence to Steffen Hering, Institut für Biochemische Pharmakologie, Peter-Mayr-Straße 1, A-6020 Innsbruck, Austria. E-mail Steffen.Hering.{at}uibk.ac.at

Abstract— L-type calcium channels (Ca_v1.m) inactivate in response to elevation of intracellular Ca²⁺ (Ca²⁺-dependent inactivation) and additionally by conformational changes induced by membrane depolarization (fast and slow voltage-dependent inactivation). Molecular determinants of inactivation play an essential role in channel inhibition by phenylalkylamines (PAAs). The relative impacts, however, of Ca²⁺-dependent and voltage-dependent inactivation in Ca_v1.2 sensitivity for PAAs remain unknown. In order to analyze the role of the different inactivation processes, we expressed Ca_v1.2 constructs composed of different ß-subunits (ß_1a-, ß_2a-, or ß₃-subunit) in Xenopus oocytes and estimated their (-)gallopamil sensitivity by means of the two-microelectrode voltage clamp with either Ba²⁺ or Ca²⁺ as charge carrier. Ca_v1.2 consisting of the ß_2a-subunit displayed the slowest inactivation and the lowest apparent sensitivity for the PAA (-)gallopamil. A significantly higher apparent (-)gallopamil-sensitivity with Ca²⁺ as charge carrier was observed for all 3 ß-subunit compositions. The kinetics of Ca²⁺-dependent inactivation and slow voltage-dependent inactivation were not affected by drug. The higher sensitivity of the Ca_v1.2 channels for (-)gallopamil with Ca²⁺ as charge carrier results from slower recovery (_rec,Ca 15 seconds versus _rec,Ba 3 to 5 seconds) from a PAA-induced channel conformation. We propose a model where (-)gallopamil promotes a fast voltage-dependent component in Ca_v1.2 inactivation. The model reproduces the higher drug sensitivity in Ca²⁺ as well as the lower sensitivity of slowly inactivating Ca_v1.2 composed of the ß_2a-subunit.

Key Words: Ca_v1.2 • ß-subunit • Ca²⁺-dependent inactivation • voltage-dependent inactivation • PAA (-)gallopamil

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