Regulation of Ca2+-activated nonselective cationic currents in rat pituitary GH3 cells: involvement in L-type Ca2+ current.

Ionic currents were investigated by a patch clamp technique in a clonal strain of pituitary (GH3) cells, using the whole cell configuration with Cs+ internal solution. Depolarizing pulses positive to 0 mV from a holding potential of -50 mV activated the voltage-dependent L-type Ca2+ current (ICa,L) and late outward current. Upon repolarization to the holding potential, a slowly decaying inward tail current was also observed. This inward tail current upon repolarization following a depolarizing pulse was found to be enhanced by Bay K 8644, but blocked by nifedipine or tetrandrine. This current was eliminated by Ba2+ replacement of external Ca2+ as the charge carrier through Ca2+ channels, removal of Ca2+ from the bath solution, or buffering intracellular Ca2+ with EGTA (10 mM). The reversal potential of inward tail current was approximately -25 mV. When intracellular Cl- was changed, the reversal potential of the Ca2+-activated currents was not shifted. Thus, this current is elicited by depolarizing pulses that activate ICa,L and allow Ca2+ influx, and is referred to as Ca2+-activated nonselective cationic current (ICAN). Without including EGTA in the patch pipette, the slowly decaying inward current underlying the long-lasting depolarizing potential after Ca2+ spike was also observed with a hybrid current-voltage protocol. Thus, the present studies clearly indicate that Ca2+-activated nonselective cationic channels are expressed in GH3 cells, and can be elicited by the depolarizing stimuli that lead to the activation of ICa,L.