Effects of dichlorobenzamil on calcium currents in clonal GH3 pituitary cells

Membrane currents were recorded from voltage-clamped clonal (GH3) pituitary cells, using the whole-cell patch clamp method. Under conditions in which currents through Na+ and K+ channels were abolished, two distinct Ca++ channel currents were identified. Dichlorobenzamil (DCB), an amiloride analog and potent inhibitor of Na-Ca exchange, inhibited both the T-type Ca++ current (ICa,t) and slowly-inactivating L-type current carried by either Ca++ (ICa,s) or Ba++ (IBa,s). The blockade was dose-dependent (1-25 microM) and ICa,t was more sensitive to inhibition by DCB than the L-type channel currents. Although the inhibition of ICa,t was not accompanied by changes in the time course of inactivation of the T-type channels, the blockade of L-type Ca++ channel currents was time-dependent, increasing throughout the depolarizing test pulse (300 msec). Repetitive stimulation at 1.0 Hz or the application of depolarizing prepulses augmented the blockade of ICa,s and IBa,s by DCB. It is proposed that the blockade is modulated by the functional state of the L-type channel, being enhanced by the channel opening. Currents conveyed by Na+ ions through both types of Ca++ channels in GH3 cells equilibrated with Ca++-free salines were inhibited by DCB concentrations (1-5 microM) comparable to those required for blocking the currents conveyed by Ca++ or Ba++ ions.