Inactivation of calcium channels in vascular smooth muscle myocytes

Abstract

Many of the structural domains involved in Ca2+ channel (CACN) inactivation are also involved in determining their sensitivity to antagonist inhibition. We hypothesize that differences in inactivation properties and their structural determinants may suggest candidate domains as targets for the development of novel, selective antagonists. The characteristics of Ca2+ current (ICa) inactivation, steady-state inactivation (SSIN), and recovery from inactivation were studied in freshly dispersed smooth muscle cells from rabbit portal vein (RPV) using whole-cell, voltage-clamp methods. The time course of inactivation could be represented by two time constants. Increasing ICa by increasing [Ca2+]o or with more negative holding potentials decreased both time constants. With Sr2+, Ba2+, or Na+ as the charge carrier, ICa inactivation was also represented by two time constants, both of which were larger than those found with Ca2+. With Ca2+, Sr2+, or Ba2+ as the charge carrier, both time constants had minimum values near the voltage associated with maximum current. When Na+ (140 mM) was the charge carrier, voltages for Imax (-20 mV) or taumin (0 mV) did not correspond. SSIN of ICa had a half-maximum voltage of -32 +/- 4 mV for Ca2+, -43 mV +/- 5 mV for Sr2+, -41 +/- 5 mV for Ba2+, and -68 +/- 6 mV for Na+. The slope factor for SSIN per e-fold voltage change was 6.5 +/- 0.2 mV for Ca2+, 6.8 +/- 0.3 for Sr2+, and 6.6 +/- 0.2 for Ba2+, representing four equivalent charges. When Na+ or Li+ was the charge carrier, the slope factor was 13.5 +/- 0.7 mV, representing two equivalent charges. For ICa in rat left ventricular (rLV) myocytes, there was no difference in the slope factor of SSIN for Ca2+ and Na+. The rate of recovery of ICa from inactivation varied inversely with recovery voltage and was independent of the charge carrier. These results suggest that inactivation of ICa in PV myocytes possess an intrinsic voltage dependence that is modified by Ca2+. For RPV but not rLV ICa, the charge of the permeating ion confers the voltage-dependency of SSIN.