Calcium-dependent Inactivation of Potential-dependent Calcium Inward Current in an Isolated Guinea-pig Smooth Muscle Cell

1. Calcium current (ICa) was studied in single isolated smooth muscle cells of a guinea-pig taenia caeci dialysed with Cs+-containing solution to suppress K+ outward current. 2. With increasing step depolarizations up to +10 mV, acceleration of ICa inactivation was observed. With further increase of step depolarization, ICa inactivation was slowed down. The largest ICa (observed at +10 mV) was characterized by the maximal speed of inactivation. 3. Comparison of ICa in different external concentrations of Ca2+ ions ([Ca2+]o) revealed that at the same membrane potential the time course of ICa inactivation was slower, the smaller the amplitude of ICa. Slowing down of ICa inactivation was observed also during its partial block by Co2+ ions. 4. Elevation of temperature increased ICa peak amplitude and accelerated its decay. The amplitude of ICa was increased by a factor of 1.7 +/- 0.14 (n = 6) when the temperature was raised by 10 degrees C. 5. Calculations of Ca2+ entry during ICa as a time integral of Co2+-sensitive current, and comparison with the degree of ICa inactivation, showed that inactivation was tightly related to Ca2+ entry in the membrane potential range -20 to +40 mV. 6. Ba2+ current through Ca2+ channels was larger than ICa and its inactivation was considerably slower. 7. Recovery of ICa from inactivation was found to be potential dependent. When the cell membrane was hyperpolarized, ICa recovery was accelerated. 8. It was concluded that inactivation and recovery of ICa in smooth muscle cells were influenced by both Ca2+ entry and membrane potential. It was also pointed out that the observed events are difficult to explain by the hypothesis that inactivation was produced simply by accumulation of Ca2+ ions near the inner side of the membrane, and that recovery was due to lowering of internal free Ca2+ ion concentration ([Ca2+]i).