A NEW APPROACH TO SEPARATION OF VOLTAGE-ACTIVATED CA CURRENTS IN RAT MELANOTROPHS

The whole cell patch clamp technique was used to study voltage-activat ed calcium currents in rat melanotrophs. Voltage ramps of the same amp litude and of different durations were applied to separate Ca currents with different electrophysiological properties. The application of th ose with voltage gradient of 0.5 V/s resulted in a clear separation of two inward current components, activated at -55+/-5 mV and -24+/-5 mV respectively, corresponding to low (LVA) and high (HVA) voltage-activ ated currents. The proportion of cells displaying such current separat ion was reduced either at voltage gradients of 0.053 V/s or 1.6 V/s. T he application of nitrendipine and nicardipine resulted in a blockage of both currents, where the HVA current component was more sensitive t o the agents. The sensitivity of both Ca currents to dihydropyridine a ntagonists suggests that these currents may be mediated by subtypes of a dihydropyridine sensitive Ca channel.