On the role of Ca(2+)- and voltage-dependent inactivation in Ca(v)1.2 sensitivity for the phenylalkylamine (-)gallopamil

L-type calcium channels (CA(v)1.m) inactivate in response to elevation of intracellular CA(2+) (CA(2+)-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 CA(2+)-dependent and voltage-dependent inactivation in CA(v)1.2 sensitivity for PAAs remain unknown. In order to analyze the role of the different inactivation processes, we expressed CA(v)1.2 constructs composed of different beta-subunits (beta(1a)-, beta(2a)-, or beta(3)-subunit) in Xenopus oocytes and estimated their (-)gallopamil sensitivity by means of the two-microelectrode voltage clamp with either Ba(2+) or CA(2+) as charge carrier. CA(v)1.2 consisting of the beta(2a)-subunit displayed the slowest inactivation and the lowest apparent sensitivity for the PAA (-)gallopamil. A significantly higher apparent (-)gallopamil-sensitivity with CA(2+) as charge carrier was observed for all 3 beta-subunit compositions. The kinetics of CA(2+)-dependent inactivation and slow voltage-dependent inactivation were not affected by drug. The higher sensitivity of the CA(v)1.2 channels for (-)gallopamil with CA(2+) as charge carrier results from slower recovery (tau(rec,CA) approximately 15 seconds versus tau(rec,Ba) approximately 3 to 5 seconds) from a PAA-induced channel conformation. We propose a model where (-)gallopamil promotes a fast voltage-dependent component in CA(v)1.2 inactivation. The model reproduces the higher drug sensitivity in CA(2+) as well as the lower sensitivity of slowly inactivating CA(v)1.2 composed of the beta(2a)-subunit.