Nitrendipine and Ω-conotoxin modulate gonadotropin release and gonadotrope [Ca²⁺]_i

We have examined the pharmacology of the voltage-sensitive Ca²⁺ channels (VSCCs) that mediate gonadotropin secretion from primary cultures of rat pituitary cells, stimulated by either cell depolarization or by binding of gonadotropin-releasing hormone (GnRH). We also measured single-cell [Ca²⁺]_i transients using fura-2 in gonadotropes identified by a reverse hemolytic plaque assay employing an antiserum to luteinizing hormone (LH). Cell depolarization evoked by either 50 mM K⁺ or 30 μM veratridine induced 2- to 6-fold increases in gonadotropin secretion over basal levels. GnRH caused 6- to 20-fold increases in follicle-stimulating hormone (FSH) and LH secretion, respectively, with maximal stimulation at 100 nM GnRH. K⁺- or GnRH-induced FSH release was largely prevented by co-incubation with 1 mM CdCl. Tetrodotoxin (TTX, 5 μM) prevented the veratridine-, but not the K⁺- or GnRH-induced, stimulation of FSH secretion. Nitrendipine (Ntd, 1 μM) produced 35-50% inhibition (NS) of both FSH and LH release stimulated by either 50 mM K⁺ or 100 nM GnRH. Ntd also inhibited the K⁺-induced [Ca²⁺] _i rise (>90%), as well as the secondary, plateau phase of the GnRH-induced elevation of [Ca²⁺]_i (100% inhibition). Omega-conotoxin (Ω-CgTx, 100 nM) partially suppressed FSH and LH release (NS) due to both K⁺ (33% each) and GnRH (44% and 18%, respectively). Ω-CgTx showed variable effects on [Ca²⁺]_i transients evoked by K⁺ or GnRH ranging from clear inhibition to no effect. We conclude that influx of extracellular Ca²⁺ is one of several fundamental events underlying the depolarization- or receptor-activated release of LH and FSH, and that this influx can be inhibited by dihydropyridine-sensitive ('L') Ca²⁺ channels. Two classes of L-channels may exist in gonadotropes, that differ in their sensitivity to Ω-CgTx.