Luteinizing Hormone and Prolactin Release by Cultured Pituitary Cells and by Gonadotrope- and Lactotrope-Enriched Populations in the Presence of Modulators of Adenylate Cyclase

Abstract

The effects of 3 compounds previously known to alter the adenylate cyclase activity in various tissues were tested on rat pituitary. Forskolin increased while RMI 12,330 A and SQ 22,536 decreased enzyme activity of pituitary homogenates. Forskolin (10^–5M) promoted in cultured cells a massive synthesis of cyclic adenosine monophosphate (cAMP) and intense output of the nucleotide in the medium; at the same concentration, RMI 12,330 A decreased the cell content of cAMP while release was unaffected, and SQ 22,536 exerted no effect on the cyclic nucleotide amount of both cell and medium. The basal release of plated total cells or enriched gonadotropes was slightly stimulated by forskolin (10^–7–10^–5M) though the level of stimulation never attained the one produced by 10^–7M luteinizing hormone-releasing hormone (LHRH); it was affected neither by RMI 12,330 A (10^–7–10^–5M) nor by SQ 22,536 (10^–7–10^–4M). The LHRH-stimulating effect was slightly amplified during the 1st h (sensitivity response of cells) by low concentrations of forskolin (10^–7 and 10^–6M) but the overall luteinizing hormone (LH) release after 4 h of incubation (capacity response of cells) was unmodified. Higher concentrations of the diterpene inhibited the LHRH effect as seen with total cells and with enriched gonadotropes. RMI 12,330 A at 10^⁵M significantly inhibited the LHRH-promoted LH release from total cells and from gonadotropes but a higher concentration (10^–4M) promoted a large unspecific release that masked the LHRH effect. SQ 22,536 never modified the cell response to LHRH, whatever the drug concentration. We thus conclude that LH release is partly cAMP dependent, the cyclic nucleotide acting during the very early phase of LHRH action (on the readily releasable LH pool, i.e. sensitivity response) but exerting no effect during the 2nd step (mobilization of LH stores, i.e. capacity response). Forskolin (10^–7 10^–5M) stimulated prolactin (PRL) release, but it was unable to overcome the dopamine-(DA-)inhibiting effect though the adenylate cyclase was stimulated by 10^–6M of the diterpene in the presence of 10^⁷ M DA. 10^–5M of RMI 12,330 A inhibited PRL release as efficiently as 10^–7M DA did. SQ 22,536 exerted no effect, whatever the drug concentration. These results confirm the participation of the cAMP-generating system in PRL release; however, activation of the adenylate cyclase is not sufficient to stimulate hormone release once other membrane events are blocked.