CALCIUM CURRENTS IN THE A7R5 SMOOTH MUSCLE-DERIVED CELL-LINE - INCREASE IN CURRENT AND SELECTIVE REMOVAL OF VOLTAGE-DEPENDENT INACTIVATION BY INTRACELLULAR TRYPSIN

We studied the effects of trypsin on L-type calcium current in the A7r5 smooth muscle cell line. Intracellular dialysis with trypsin increased the whole-cell current up to fivefold. The effect was concentration dependent, and was prevented by soybean trypsin inhibitor. Ensemble analysis indicated an increase in the number of functional channels, and possibly a smaller increase in the open probability, with no change in the single channel current. The shape of the current-voltage curve was unaffected. Trypsin also nearly eliminated inactivation of currents carried by Ba2+, but had little or no effect on the rapid inactivation process in Ca2+. This indicates that trypsin removes voltage-dependent but not Ca2+-dependent inactivation, suggesting the existence of distinct protein domains for these two mechanisms of calcium channel inactivation.