A NOVEL CALCIUM-ACTIVATED APAMIN-INSENSITIVE POTASSIUM CURRENT IN PITUITARY GONADOTROPHS

In cultured rat pituitary gonadotrophs, GnRH-induced oscillations in c ytosolic calcium concentration ([Ca2+](i)) are associated with periodi c membrane hyperpolarization. The hyperpolarizing waves are secondary to the activation of apamin-sensitive Ca2+-activated K+ channels that account for a single class of I-125-apamin binding sites present in th ese cells. In a substantial fraction of gonadotrophs, however, we obse rved a Ca2+-controlled oscillatory current that was resistant to apami n, even at concentrations five orders of magnitude higher than the dis sociation constant (K-d) observed in the binding experiments. With the K+ in the pipette, the apamin-resistant current showed a reversal pot ential of -42 mV, nearly 40 mV more positive than that of the apamin-s ensitive current. With Cs+ in place of K+ in the pipette solution, bot h the size of the apamin-insensitive current and its reversal potentia l remained unchanged. Ion substitution studies further revealed that t he reversal potential was independent of Cl-, In contrast, an 11 mV hy perpolarizing shift in the reversal potential occurred when extracellu lar Na+ was reduced to 80 mM. In cells expressing apamin-resistant con ductances, addition of apamin evoked a marked increase in the duration of the action potentials and reduction in the frequency of spontaneou s spiking. In the presence of GnRH, gonadotrophs exhibit the typical b urst pattern of electrical activity. Further exposure of the cells to apamin depolarized the membrane from a silent phase bursting level of about -80 mV to a new level of about -40 mV. These observations indica te that, in addition to apamin-sensitive current, a subpopulation of p ituitary gonadotrophs also expresses a cationic component of the Ca2+- activated membrane conductance that has the potential to remodulate sp ontaneous and agonist-induced electrical activity.