Gonadotropin-releasing hormone modifies action potential generation in sheep pars distalis gonadotropes

Cell-intact patch-clamp recording was used to determine the electrophysiological responses of sheep anterior pituitary gonadotropes to stimulation with gonadotropin-releasing hormone (GnRH). Cells were identified prior to recording by reverse haemolytic plaque assay (RHPA), or using morphological criteria in preparations enriched in gonadotropes by Percoll density gradient centrifugation. Most cells identified by RHPA did not generate action potentials, and responses to GnRH were inconsistent. The majority of gonadotropes in enriched preparations however spontaneously generated action potentials requiring the entry of both extracellular Na+ and Ca2+, and involving tetraethylammonium-sensitive K+ channels. Two-minute GnRH application (10(-7) M) evoked a characteristic sequence of changes in action potential generation. The immediate response was an inhibition of action potentials, followed by a recovery of these events, with a progressive decline in amplitude over about 10 min. The cells then remained quiescent for up to 1 h. The results indicate that GnRH may evoke an initial hyperpolarization involving Ca(2+)-dependent K+ channels, followed by a sustained depolarizing response, with consequent inactivation of action potential generation.