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 (I(Ca,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 I(Ca,L) and allow Ca²⁺ influx, and is referred to as Ca²⁺- activated nonselective cationic current (I(CAN)). 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 I(Ca,L).