Estradiol modulation of PMA--and ionomycin-stimulated LH secretion from pituitaries of castrated rats

Adenosine 3',5'-cyclic monophosphate and the self-priming effect of gonadotrophin-releasing hormone

An in vitro perifusion system was used to ascertain the role of cAMP in the genesis of the self-priming effect of gonadotrophin-releasing hormone (GnRH) in rat pituitaries. Ten-minute pulses of 20 nmol/l GnRH administered 150 min apart resulted in the manifestation of the self-priming effect, an effect which was inhibited by 5 mumol/l cycloheximide. Forskolin (1 mumol/l) which does not stimulate luteinizing hormone (LH) secretion or affect the initial LH response to GnRH significantly potentiated the second response through protein synthesis-dependent mechanisms. Additionally, an initial 10-min pulse of forskolin alone was sufficient to prime the pituitary to a subsequent pulse of GnRH 150 min later. Interestingly, similar amounts of LH were secreted in response to forskolin + GnRH or GnRH administered 150 min after forskolin. Flufenamate, an inhibitor of GnRH-stimulated increases in cAMP production prevented the manifestation of the self-priming effect of GnRH. Forskolin which bypasses the inhibitory effects of flufenamate on cAMP production reversed the flufenamate-induced inhibition of the self-priming effect of GnRH through protein synthesis-dependent processes. These results suggest that cAMP does not mediate the LH response to an initial exposure of GnRH, but does play a pivotal role in the genesis of the self-priming effect of GnRH through the stimulation of de novo protein synthesis. Once the newly synthesized proteins are available, the nucleotide is not required for the manifestation of the phenomenon.

The involvement of dihydropyridine-sensitive calcium channels in phorbol ester-induced luteinizing hormone and growth hormone release

We examined the role of voltage-activated, L-type, Ca2+ channels in phorbol ester-induced luteinizing hormone (LH) and growth hormone (GH) release from rat anterior pituitary tissue. The L-type Ca2+ channel inhibitor, nimodipine (NMD), inhibited phorbol 12,13-dibutyrate (PDBu)-induced GH release but had no significant effect on LH release. The L-type Ca2+ channel activator BAY K 8644 had no effect on PDBu-induced GH release but potentiated PDBu-induced LH release. In contrast, 60 mM K(+)-induced LH and GH release were inhibited by NMD, whereas BAY K 8644 had no effect. When PDBu and either K+ or BAY K 8644 were used together, they acted synergistically to evoke levels of LH release greater than addition of release caused by each secretagogue alone. However, the release of GH was additive with PDBu and either K+, BAY K 8644. The protein kinase C (PKC) inhibitor staurosporine inhibited both PDBu-induced LH release and GH release. A structurally different PKC inhibitor, H7, significantly inhibited PDBu-induced LH release but had no effect on PDBu-induced GH release. Both staurosporine and H7 inhibited LH release induced by PDBu and BAY K 8644 together. In contrast, although staurosporine inhibited GH release induced by PDBu and BAY K 8644, H7 significantly potentiated this response. A difference in the action of these two inhibitors was also apparent on K(+)-induced hormone release where staurosporine partially blocked K(+)-induced LH and GH release but H7 had no effect on the release of either hormone. Data obtained in 45Ca2+ influx experiments further suggested that a staurosporine-sensitive, but H7-resistant, PKC-like kinase may tonically maintain L-channels in a voltage-sensitive state, as down-regulation of PKC in dispersed anterior pituitary cells by long term PDBu treatment caused a significant reduction in K(+)-induced 45Ca2+ influx. We conclude that phorbol ester-induced GH release, but not LH release, is a result of L-type Ca2+ channel activation which may occur by means of alterations in the channel itself to increase its responsiveness to a given depolarisation.

The differential effects of protein kinase C activators and inhibitors on rat anterior pituitary hormone release

We investigated the possibility that various protein kinase C (PKC) activators and inhibitors may differentially affect luteinizing hormone (LH) and growth hormone (GH) release from rat anterior pituitary tissue, incubated in vitro. Activators of PKC induced LH release with the following order of potency: mezerein > phorbol 12,13-dibutyrate (PDBu). Mezerein and PDBu were equipotent on GH release. A range of PKC inhibitors (including compounds highly selective for PKC) potently and completely inhibited PKC activator-induced LH and GH release. Chelerythrine and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H7) were less potent inhibitors of PDBu-induced GH release than of LH release. A component of PDBu- and mezerein-induced LH release was inhibited by H7 with high potency, but a second H7-insensitive component was detected. Mezerein- and PDBu-induced GH release consisted of an H7-resistant component only. When the regulatory domain of PKCs from different sources was investigated by displacement of [3H]PDBu binding, the affinity for mezerein was 3-5-fold greater than that for PDBu at PKCs from cerebral cortex, lung and alpha and beta isoforms extensively purified from brain. Anterior pituitary PKCs were unusual in showing closely matched affinity for mezerein and PDBu, reminiscent of their equivalent potency on GH release. In order to investigate the potency of the catalytic domain inhibitor H7 on PKCs from different sources, enzyme activity assays were carried out on partially purified cytosolic PKCs from midbrain and anterior pituitary and on extensively purified PKC alpha and PKC beta. The Ca(2+)-independent component of PDBu-induced (phosphatidylserine-dependent) activity from anterior pituitary alone showed unusually low potency of inhibition by H7 but was potently inhibited by staurosporine and Ro 31-8220. In contrast, the Ca(2+)-dependent PKC activity in anterior pituitary was inhibited by H7, staurosporine and Ro-31-8220 with high potency as in all other preparations. These results are consistent with the presence and active role in secretion of pharmacologically distinct forms of PKC (or PKC-like kinases) in rat anterior pituitary cells.