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Department of Pharmacology and Toxicology, Michigan State University, East Lansing 48824.
1. Currents through Ca2+ channels of single guinea-pig ventricular myocytes were studied using patch electrodes for whole-cell recording. Currents through Na+ and K+ channels were suppressed by the application of drugs or the substitution of impermeant ions. 2. Inactivation of the Ca2+ current (ICa) was investigated using a two-pulse protocol. The amount of inactivation left behind by a pre-pulse appeared to be related to current magnitude, as others have reported. The dependence of inactivation on the pre-pulse potential was partially U-shaped, as the amount of inactivation peaked at 0 mV and then declined with more positive pre-pulses. 3. Non-specific current carried by monovalent ions through Ca2+ channels (Ins) was induced by lowering the extracellular Ca2+ concentration with EGTA. Ins peaked in an inward direction at -20 mV, reversed direction at +22 mV, and became a large outward current at more positive potentials. 4. Ins inactivated with a slow time course. The inactivation was not due to accumulation or depletion phenomena. Studies using two-pulse protocols showed that the amount of inactivation left by a pre-pulse was directly related to the pre-pulse potential. 5. The addition of micromolar amounts of free Ca2+ to the external solution induced outward rectification of Ins. Inward currents were small or absent, while larger outward currents could still be seen at very positive potentials. Ca2+-channel inactivation still occurred under these conditions, even in the absence of any significant ionic movement. 6. The time courses of Ins inactivation and recovery were studied. The half-time of Ins inactivation decreased with larger depolarizations. Recovery of Ins was very slow, but could be accounted for by changes in the surface charge of the membrane. 7. It is concluded that Ins inactivation is due solely to a voltage-dependent inactivation process which is intrinsic to myocardial Ca2+ channels. Voltage-dependent inactivation appears to account for a significant proportion of total Ca2+-channel inactivation at negative potentials, and appears to account for almost all of the inactivation at very positive potentials, even in the presence of millimolar concentration of external Ca2+.
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