PROPERTIES OF INWARD CALCIUM CURRENT IN GUINEA-PIG URETERAL MYOCYTES

Ureteral myocytes of guinea pigs have L-type Ca2+ channels (I-Ca). In 3 mM Ca2+, maximum I-Ca was 3.38 mu A/cm(2). Voltage at which conducta nce is 50% of maximum (V-0.5) Of I-Ca was -1.0 mV in 3 mM Ca2+ and +22 mV in 30 mM Ca2+, with slope factors of 8 mV. V-0.5 Of steady-state i nactivation of I-Ca was -16.2 and +1.1 mV in 3 and 30 mM Ca2+, respect ively, with similar slope factors of about -6 mV. A window current rea ching 20-25% of the maximum I-Ca was active between -20 and 0 mV. I-Ca inactivated very slowly, with time constants of 217.6 and 2,455.9 ms with no voltage dependency. When Ba2+ was used as tile charge carrier, the amplitude and inactivation kinetics of the Ba2+ current were simi lar to those for I-Ca. These results indicate that the ureteral myocyt e has little Ca2+-mediated Ca2+ channel inactivation, a feature signif icantly associated with the slow I-Ca inactivation. The slow inactivat ion and the window current are essential for the sustained membrane de polarization during the plateau of ureteral action potentials.