A new approach to separation of voltage-activated Ca currents in rat melanotrophs.

The whole cell patch clamp technique was used to study voltage-activated calcium currents in rat melanotrophs. Voltage ramps of the same amplitude and of different durations were applied to separate Ca currents with different electrophysiological properties. The application of those 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-activated currents. The proportion of cells displaying such current separation was reduced either at voltage gradients of 0.053 V/s or 1.6 V/s. The application of nitrendipine and nicardipine resulted in a blockage of both currents, where the HVA current component was more sensitive to the agents. The sensitivity of both Ca currents to dihydropyridine antagonists suggests that these currents may be mediated by subtypes of a dihydropyridine sensitive Ca channel.