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.
Sokolov
S
S
Institut für Biochemische Pharmakologie, Innsbruck, Austria.
Timin
E
E
Hering
S
S
D000818:Animals; D001464:Barium / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020746:Calcium Channels, L-Type / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D005711:Gallopamil / Q000494:pharmacology*; D017136:Ion Transport / Q000187:drug effects; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008845:Microinjections; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D021122:Protein Subunits; D018075:RNA, Complementary / Q000008:administration & dosage / Q000235:genetics / Q000378:metabolism; D014981:Xenopus
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; C419385:L-type calcium channel alpha(1C); D021122:Protein Subunits; D018075:RNA, Complementary; D001464:Barium; D005711:Gallopamil; D002118:Calcium