Ca ²⁺-activated K ⁺ channels in gonadotropin- releasing hormone-stimulated mouse gonadotrophs

GnRH receptor activation elicits release of intracellular Ca ²⁺, which leads to secretion and also activates Ca ²⁺-activated ion channels underlying membrane voltage changes. The predominant Ca ²⁺-activated ion channels in rat and mouse gonadotrophs are Ca ²⁺-activated K ⁺ channels. To establish the temporal relationship between GnRH-induced changes in intracellular [Ca ²⁺] ([Ca ²⁺] _i, ) and membrane current (I ^m), and to identify specific Ca ²⁺-activated K ⁺ channels linking GnRH- induced increase in [Ca ²⁺], to changes in plasma membrane electrical activity, we used single female mouse gonadotrophs in the perforated patch configuration of the patch-clamp technique, which preserves signaling pathways. Simultaneous measurement of [Ca ²⁺], and Im in voltage- clamped gonadotrophs revealed that GnRH stimulates an increase in [Ca ²⁺], that precedes outward Im, and that activates two kinetically distinct currents identified, using specific toxin inhibitors, as small conductance Ca ²⁺-activated K ⁺ (SK) current (I _SK) and large (big) conductance voltage- and Ca ²⁺-activated K ⁺ (BK) current (I _BK). We show that the apamin-sensitive current has an IC50 of 69 pM, consistent with the SK2 channel subtype and confirmed by immunocytochemistry. The magnitude of the SK current response to GnRH was attenuated by 17β-estradiol (E ₂) pretreatment. Iberiotoxin, an inhibitor of BK channels, completely blocked the residual apamin-insensitive outward Im, substantiating that I _BK is a component of the GnRH-induced outward Im. In contrast to its suppression of I _SK, E ₂ pretreatment augmented peak I _BK. S _K or B _K channel inhibition modulated GnRH-stimulated LH secretion, implicating a role for these channels in gonadotroph function. In summary, in mouse gonadotrophs the GnRH-stimulated increase in [Ca ²⁺], activates I _SK and I _BK, which are differentially regulated by E ₂ and which may be targets for E ₂ positive feedback in LH secretion.