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.
Josephson
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Department of Physiology and Biophysics, University of Cincinnati, College of Medicine, Ohio 45267.
Sperelakis
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N
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology*; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D004594:Electrophysiology; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D009206:Myocardium / Q000378:metabolism*
D015220:Calcium Channels; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester