No abstract

Interleukin-6 (IL-6), a cytokine produced by inflammatory reactions, was found to stimulate PRL, GH and LH release from anterior pituitary cells at concentrations similar to those which affected lymphocyte mitogenesis. Perifused pituitary cells responded to IL-6 with prompt increases in hormone release that declined rapidly following cessation of exposure. Dopamine (DA) attenuated IL6-induced PRL release. In addition, IL-6 potentiated both GHRF- and TRH-induced hormone release without an affect on intracellular cAMP. These data demonstrate a new biological activity for IL-6 and provide evidence for immune system regulation of anterior pituitary hormone release.

No abstract

Therapy for large prolactinomas remains controversial. Surgery is often unsuccessful in restoring endocrine function to normal. However, medical therapy with bromocriptine, a dopamine agonist, not only suppresses PRL levels, but may also lead to a reduction in tumor size. Previous reports have demonstrated radiographic evidence of tumor regression only after 3 or more months of bromocriptine therapy. We have now documented, for the first time, objective evidence of extremely rapid reduction in tumor size in two patients harboring large PRL-secreting pituitary tumors (mean pretreatment serum PRL levels, 2350 and 3900 ng/ml) who were prospectively treated with bromocriptine (7.5 mg/day) in preference to surgical intervention despite marked visual impairment in one of the patients. After 2 and 6 weeks of therapy, respectively, marked reduction in tumor size was demonstrated radiographically in both patients. Headache, visual acuity, and visual fields had improved after only 3 days. Although the mechanism of bromocriptine's antitumor activity is unclear, we believe that a large prospective trial to study the effects of bromocriptine therapy on the size of PRL-secreting macroadenomas is urgently needed to determine whether medical therapy should become the primary modality of treatment to reduce tumor size as well as restore endocrine function.

We recently reported that the cytokine interleukin-6 (IL-6) is a potent stimulator of anterior pituitary hormone release in vitro. Since IL-6 is not normally detectable in the blood, we hypothesized that IL-6 may be produced by the anterior pituitary in situ and thereby affect hormone secretion through paracrine or autocrine mechanisms. The present study demonstrates that cultured anterior pituitary cells spontaneously secrete large quantities of IL-6 in vitro. IL-6 was detectable in the incubation medium within 2 h, and by 8 h of culture had attained concentrations of 2000-4000 U/ml.4 x 10(5) cells. IL-6 production was stimulated by phorbol myristate acetate (10-100 nM) approximately 2-fold and by lipopolysaccharide (0.001-10.0 micrograms/ml) 4-fold during 4-h incubations. In contrast, the cytokine recombinant human IL-1 alpha had no effect on IL-6 release by cultured pituitary cells. Freshly dissected hemipituitary tissue also secreted more than 3000 U/ml IL-6 during a 4-h incubation. This secretion was enhanced 3-fold by 10 micrograms/ml lipopolysaccharide. Our results suggest that the anterior pituitary may produce IL-6 in situ, where it may function as an intrapituitary releasing factor.

No abstract

Although dopamine inhibits PRL release from the normal anterior pituitary lactotroph, a conclusive demonstration of the mechanisms involved in this response has been impeded by the presence of other cell types in the anterior pituitary. To circumvent this problem, we have isolated a clonal cell line, designated MMQ, from the 7315a rat pituitary tumor. The MMQ cell is an exemplary model for our use because it only secretes PRL. Our studies show that dopamine inhibits secretagogue-induced PRL release from these cells. In addition, dopamine decreases the intracellular cAMP concentration in MMQ cells that have been exposed to forskolin, cholera toxin, or vasoactive intestinal polypeptide, each a stimulator of cAMP generation. This inhibition is, in turn, reversed by the dopamine antagonist haloperidol and by pertussis toxin, an inactivator of the GTP-binding coupling protein. Dopamine also decreases the uptake and fractional efflux of 45Ca2+ by MMQ cells that have been exposed to the calcium channel activator maitotoxin. It seems, therefore, that dopamine decreases PRL release from MMQ cells at least in part by decreasing intracellular cAMP levels and calcium uptake. In additional experiments, we have found that MMQ cells are responsive to somatostatin, estrogen, progesterone, and acetylcholine, but not to TRH, angiotensin II, neurotensin, or bombesin. Furthermore, these cells possess a functional protein kinase-C system, as evidenced by the increase in PRL release and decrease in stimulated intracellular cAMP levels that occur in response to treatment with phorbol diesters. We suggest that the MMQ cell line will prove a useful model system for study of the biochemical effects of dopamine and other factors that modify PRL release.

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.