Abstract
1. Calcium currents (ICa) were studied in voltage-clamped NCB-20 cells. In undifferentiated cells, voltage steps from hyperpolarized potentials (-80/-100 mV) essentially revealed transient ICa showing characteristics classically described for "T-type" channels. In about 50% of the cells, there was a residual current at the end of the step; no ICa was elicited from a holding potential of -50 mV. 2. In contrast, 100% of the cells differentiated with dibutyryl cyclic AMP (cAMP) displayed a residual current in addition to the transient one, and depolarizing steps from a holding potential of -50 mV induced a sustained current. In these cells, Bay K 8644 elicited both a negative shift in voltage dependence and a moderate increase of the sustained component. 3. Although these changes in Ca2+ channel physiology result from chemically induced differentiation, they might not be directly related to the concomitant morphologic differentiation. 4. In undifferentiated NCB-20 cells, T-type Ca2+ currents can be elicited in relative isolation.