Joint voltage- and calcium dependent inactivation of Ca channels in frog atrial myocardium

Ca currents have been measured from frog atrial myocardium by use of a double sucrose gap voltage clamp technique. Ca inward currents could be separated from overlapping outward currents by means of application of 10 mM 4-aminopyridine and an intracellular Cs-load. The inactivation process of the Ca currents could be separated into two parts: a fast Ca-dependent inactivation and a slow voltage-dependent inactivation. The time constant of the fast decay of the Ca currents was decreased due to increased external Ca concentrations and was U-shaped depending on the membrane potential. As indicated by twin pulse experiments inactivation of a test current following a conditioning current could be increased by increasing the charge inwardly transported during the conditioning current. Long-lasting prepulses being too weak to activate any inward current could also inactivate the test current. Recorded by a prepulse protocol steady-state inactivation curves were described by a half maximum inactivation at -36.7 mV and a slope of 4.2 mV. All the results refer on a joint voltage and calcium dependent inactivation controlling the inwardly transported Ca via the Ca channels.