Calcium Rather Than Cyclic AMP as the Physiological Intracellular Regulator of Prolactin Release

Abstract

Studies on the mechanisms which govern the release of prolactin were undertaken using two in vitro techniques. A dispersed preparation of rat anterior pituitary cells was made by mechanical means in the presence of trypsin. These washed cells were drawn up into a small column together with a Bio-Gel matrix and perifused with Earle’s basic salt solution. The eluates containing prolactin were then collected at short intervals. Test substances were added to the perifusion medium and their effect on prolactin release was measured. The results of these studies were compared with those obtained by incubating hemipituitary glands in Medium 199 to measure the effect of test substances on the release of radioimmunoassayable prolactin. Perifusion of dispersed pituitary cells with dopamine produced a marked inhibition of prolactin release within 3 min, and maximal suppression was noted 11 min after initiating the perifusion. Upon withdrawal of dopamine, prolactin release began to recover within 1 min and continued to rise to 80% of baseline at 6.5 min. Perifusion of pituitary cells in medium free of calcium also produced a marked reduction in prolactin release which was restored after reexposure of the cells to calcium. The addition of manganese and D-600, agents which block calcium channels, also caused reversible inhibition of prolactin release. The effects of the ionophores A23187 and X537A on prolactin release were studied. The presence of calcium ionophore A23187 did not effect prolactin release but it reversed the dopamine-mediated inhibition of prolactin release. In the absence of calcium, both ionophores stimulated release of prolactin. Tetrodotoxin, a blocker of sodium channels had no effect on prolactin release. Agents such as prostaglandin E₁, and cholera toxin increased cyclic AMP levels, but no positive correlation was obtained on prolactin release patterns. Gpp(NH)p-stimulated adenylate cyclase activity in homogenates of anterior pituitary tissue was unaffected by dopamine. In contrast, addition of dibutyryl cyclic AMP to perifused pituitary glands stimulated prolactin release and theophylline added to hemipituitary gland completely reversed the inhibitory effect of dopamine on prolactin release and caused a concomitant increase in cyclic AMP levels. It is suggested that the tonic high level of prolactin release is maintained by influx of extracellular calcium and that dopamine inhibits this process. The role of intracellular cyclic AMP is undefined; however, the effects of dibutyryl cyclic AMP and theophylline may be due to mobilization of intracellular calcium and thereby stimulate prolactin release by this mechanism rather than through cyclic AMP. In summary, we present evidence that regulation of prolactin secretion by normal lactotropes is a calcium-mediated process.

Somatotroph Hyperplasia

“…Deuben (37,38), while our GRF extract demonstrated a dose-related increase in activity from 2 to 66 ng peptide/ml incubation medium, which suggests that it is active in the nanomolar range (Fig. 8) (39,40) (Fig. 8).…”

Phorbol Ester-Induced LH Release in Pituitary Gonadotrophs: Effects of Antagonists of Calmodulin and GnRH.

“…THE ACTIONS of many peptide hormones are mediated by calcium [1][2][3][4][5][6][7][8] and the mechanism of action of GnRH on LH release also appears to involve calcium-dependent processes rather than cyclic nucleotides [9,10] . Recent studies have shown that the GnRH-induced response of cytosolic Ca2+ concentration ([Ca2+]i) is closely correlated with the secretory response [11] and that calcium influx through selective calcium channels is important in regulating the GnRH-induced…”

Lack of Effect of Histidyl-Proline Diketopiperazine on Basal Level and TRH-Induced Response of Cytosolic Calcium Concentration in Rat Lactotrophs.

“…The action of TRH is mediated by calcium [14] and intracellular calcium-dependent processes play an important role in the mechanism of PRL release [15][16][17]. The TRH-stimulated increase in the cytosolic Ca2+ concentration ([Ca2+]i) is a major regulator of the secretory response [18].…”