VOLTAGE-ACTIVATED CURRENTS THROUGH CALCIUM CHANNELS IN RAT PITUITARY MELANOTROPHIC CELLS

Voltage-gated currents through Ca channels in rat melanotrophs in short-term primary culture were investigated in whole cell voltage clamp recordings, using Ba2+ ions as charge carrier. At potentials higher than - 50 mV a transient current (I(t)) was activated, which inactivated entirely during a 150-ms command pulse and reached a maximal amplitude at -10 mV. At potentials higher than -20 mV a second component (I(s)) appeared, which inactivated slowly and had its maximal amplitude between 0 and + 10 mV. The relative amplitudes of I(t) and I(s) differed between cells. Application of 40-mu-M NiCl2 reversibly blocked I(t) while leaving I(s) intact, whereas 20-mu-M CdCl2 reversibly blocked I(s), but not I(t). Nifedipine blocked I(s) concentration dependently, but did not affect I(t). The steady state inactivation of I(t) occurred between -80 and -40 mV. The steady-state inactivation curve of I(s) was located between -60 and 0 mV. The latter inactivation curve showed two phases, which were fitted best by the sum of two Boltzmann equations, suggesting the existence of two populations of I(s) channels.