INACTIVATION OF CALCIUM CHANNELS IN SINGLE VASCULAR AND VISCERAL SMOOTH-MUSCLE CELLS OF THE GUINEA-PIG

Inactivation of currents carried through calcium channels by calcium (I(Ca)), barium (I(Ba)) and monovalent cations (I(n.s.)) was studied in single smooth muscle cell (SMC) of the guinea-pig coronary artery and taenia caeci by the whole-cell patch-clamp method. The rate of I(Ca) inactivation in the coronary artery SMC was correlated with I(Ca) amplitude, and acceleration was observed with the increasing I(Ca) peak amplitude. The availability curve of I(Ca) in double-pulse experiments was found to be U-shaped, however, no complete restoration of I(Ca) availability was observed. Inactivation of I(Ba) was considerably slowlier than that of I(Ca). These findings may indicate that inactivation of calcium channels in the membrane of coronary artery SMC is, at least partially, a Ca-dependent process. However, some facts observed contradict the validity of this hypothesis for coronary artery SMC in contrast to taenia caeci: 1) elevation of external Ca2+ concentration did not affect the time course of I(Ca) inactivation; 2) inactivation of I(n.s.), i.e. without calcium entry into the cell, was faster than that of I(Ca). It was concluded that the characteristics of Ca channel inactivation were changed by the removal of divalent cations from extracellular solution. Differences and similarities in Ca channel inactivation between coronary artery and taenia caeci SMC are discussed.