Fast activation of cardiac Ca++ channel gating charge by the dihydropyridine agonist, BAY K 8644

Nonlinear charge movement (gating current) was studied by the whole-cell patch clamp method using cultured 17-d-old embryonic chick heart cells. Na+ and Ca++ currents were blocked by the addition of 10 microM TTX and 3 mM CoCl2; Cs+ replaced K+ both intra- and extracellularly. Linear capacitive and leakage currents were subtracted by a P/5 procedure. The small size (15 microns in diameter) and the lack of an organized internal membrane system in these myocytes permits a rapid voltage clamp of the surface membrane. Ca++ channel gating currents were activated positive to -60 mV; the rising phase was not distorted due to the system response time. The addition of BAY K 8644 (10(-6) M) caused a shortening of the time to peak of the Ca++ gating current, and a negative shift in the isochronal Qon vs. Vm curve. Qmax was unchanged by BAY K 8644. The voltage-dependent shift produced by BAY K 8644 is similar to that produced by isoproterenol (Josephson, I.R., and N. Sperelakis. 1990. Biophys. J. 57:305a. [Abstr.]). The results suggest that the binding of BAY K 8466 to one or more of the Ca++ channel subunits alters the kinetics and shifts the voltage dependence of gating. These changes in the gating currents can explain the parallel changes in the macroscopic Ca++ currents