A transient effect of estrogen on calcium currents and electrophysiological responses to gonadotropin-releasing hormone in ovine gonadotropes

Episodic release of luteinizing hormone (LH) by the pituitary gland is controlled by hypothalamic gonadotropin-releasing hormone (GnRH). In the period leading up to the preovulatory surge of LH, estrogen increases the number of pituitary receptors for GnRH and sensitises the gonadotropes to GnRH. The postreceptor events that are responsible for the increase in responsiveness to GnRH are not clearly delineated, but LH release is known to be Ca2+ dependent. The present study addressed the question as to whether or not estrogen may act to modify voltage-dependent Ca2+ entry in normal gonadotropes. Primary cultures enriched in gonadotropes or somatotropes were produced from anestrous female sheep. Conventional whole-cell patch-clamp recording was used to measure inward membrane current in the absence of GnRH treatment, with and without 10 nM estradiol-17 beta (E(2)) treatment for 0 to 36 h. Nystatin-perforated whole-cell patch-clamp recording was used to record membrane voltage responses to GnRH. Ca2+ current density (I-Ca, pA/pF) began to increase after 2 h exposure to E(2), and reached peak values of about 200% of control by 16-20 h (p < 0.005), then declined. If E(2) was withdrawn at 24 h, I-Ca returned towards control values by 36 h. If E(2) treatment was continued beyond 24 h, however, I-Ca fell to about 75% of control by 36 h (p < 0.005). Actinomycin D prevented the enhancement of I-Ca. E(2) was without effect on Na+ current density in gonadotropes, or on I-Ca in somatotropes. The proportion of I-Ca carried by L-type and N-type channels in gondadotropes was not changed by E(2). Ovine gonadotropes respond to GnRH with membrane potential fluctuations driven by periodic activation of Ca2+-dependent K+ channels, and synchronised action potential generation. This response was found to be sensitive to E(2). Responses were categorised according to the pattern of activity evoked by 10 nM GnRH. Without E(2) treatment, 11/14 cells responded with oscillations and 3/14 cells responded with spiking (hyperpolarizations following single action potentials). After 20 h 10 nM E(2), just 1/14 cells responded with hyperpolarizing oscillations while 13/14 cells showed spiking activity. The predominance of the spiking pattern in E(2)-treated cells is consistent with the increased Ca2+ flux, and with enhanced LH release. We conclude that E(2) has a transient effect on gonadotropes to enhance voltage-gated Ca2+ channel function. The time-course and biphasic nature of the influence of E(2) on I-Ca may be physiologically appropriate to the preovulatory LH surge. Enhanced Ca2+ influx may participate in increased Ca2+-dependent hormone release, while the delayed inhibitory action of E(2) on I-Ca may serve to limit the duration of the surge.