Robert M. MacLeod

Calcium rather than cyclic AMP as the physiological intracellular regulator of prolactin 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 t

Prolactin and growth hormone production as influenced by catecholamines and agents that affect brain catecholamines.

The in vitro influence of dopamine on the in vitro incorporation of leucine-4,5-3H into rat pituitary gland prolactin and growth hormone was studied. This catecholamine primarily inhibited

Prolactin in human and rat serum and cerebrospinal fluid

Rats treated with haloperidol or bearing subcutaneous implants of prolactin-secreting tumors had elevated CSF prolactin levels compared to those observed in control rats. These levels were commensurate with the increased serum level of prolactin, although there appeared to be an upper limit to the CSF prolactin concentration. Patients with prolactin-secreting pituitary adenomas had elevated CSF hormone levels as compared to patients with non-endocrine neurologic disease. This obtained, regardless of whether the tumor was intra- or extrasellar in its growth. The implications for the route of entry of prolactin into CSF under both normal and abnormal conditions, and the potential role for CSF prolactin as part of a feedback regulatory system on pituitary prolactin release are discussed.

Binding of Dopamine to Bovine Anterior Pituitary Gland Membranes

[3H]-dopamine (DA) binding to bovine anterior pituitary membranes were measured using sensitive in vitro ultrafiltration and centrifugation techniques. The specific interaction of [3H]-DA with the membrane fraction reached a steady-state level within 15 min at 30 degrees C and was reversible by incubating with excess nonradioactive DA. Scatchard analysis suggests the presence of 2 sites for DA specific binding having Kd values of 4.4 x 10-10 M and 4.7 x 10-8 M, respecively. Correspondingly, the total receptor concentrations were calculated to be 336 and 2,340 pmoles/g protein. Blockade of [3H]-DA binding was produced most effectively by ergocryptine and apomorphine. Perphenazine and haloperidol were considerably less active and primozide did not compete. The importance of DA in the regulation of prolactin (Prl) secretion and the characterization of pituitary DA receptor sites are discussed.

Arachidonic Acid Metabolism and Prolactin Secretion in vitro: A Possible Role for the Lipoxygenase Products

This study was designed to investigate basal and thyrotropin-releasing hormone (TRH)-stimulated prolactin release in the presence of agents that influence arachidonic acid metabolism. Agents that decrease its production by blocking phospholipase A2 activity, i.e., quinacrine and 4-bromophenacylbromide, significantly decreased prolactin secretion from anterior pituitary glands in vitro and from dispersed pituitary cells in a perifusion column. Phospholipase A2 and phorbol myristate acetate, substances that increase intracellular concentrations of arachidonic acid, markedly stimulated prolactin release by dispersed pituitary cells and by anterior pituitary glands incubated in vitro. The involvement in prolactin secretion of arachidonic acid metabolic products produced via the lipoxygenase pathway was investigated indirectly using nordihydroguaiaretic acid (NDGA), a specific inhibitor of this enzyme. NDGA progressively (dose-related) inhibited the release of prolactin in vitro and blocked the stimulating effect of 50 nM TRH on prolactin release from hemipituitary glands. Indomethacin, a specific inhibitor of the cycloxygenase pathway, had no significant effect on basal and TRH-stimulated prolactin release. The results suggest that arachidonic acid metabolism is involved in basal and TRH-stimulated prolactin secretion and that lipoxygenase pathway products are at least partially responsible for these effects.

Zinc May Have a Physiological Role in Regulating Pituitary Prolactin Secretion

We studied the in vitro influence of physiologically relevant zinc concentrations on the pituitary synthesis and secretion of prolactin (Prl). Zinc in concentrations between 1 and 10 microM reduced Prl secretion and, to a milder extent, synthesis, but not basal or stimulated growth hormone (GH) or LH release. At a supraphysiological concentration of 100 microM, zinc markedly decreased Prl synthesis and secretion, but increased LH secretion. The ability of a physiological zinc concentration to influence Prl secretion suggests that this trace element may have a role in the in vivo regulation of Prl release.

Zinc acutely, selectively and reversibly inhibits pituitary prolactin secretion.

Perifusion of dispersed female rat pituitary cells with medium containing 50 microM zinc acetate caused an acute, sustained and rapidly reversible inhibition of prolactin (Prl) secretion. This treatment had no influence on basal release of thyrotropin stimulating hormone (TSH), luteinizing hormone (LH) or growth hormone (GH). 50 microM Zn2+ also reversibly inhibited prolactin secretion stimulated by either 50 mM K+ or 10 nM TRH, but the secretion of GH, TSH and LH which was stimulated by 50 mM K+ or stimulation of TSH by 10 nM TRH was not inhibited. Thus zinc acts in a dynamic manner to selectively influence pituitary prolactin secretion.

Prolactin secretion in Parkinson disease

We studied the dopaminergic control of lactotroph cells in the anterior pituitary of parkinsonian patients and age-matched normal subjects. The resting levels of prolactin and the TRH-induced rise in prolactin were normal in Parkinson disease. Levodopa elicited a normal suppression of prolactin concentrations in parkinsonian subjects; the major abnormality to emerge was attenuation of the response to thyrotropin-releasing hormone (TRH) in the parkinsonian patients following administration of Sinemet (levodopa plus carbidopa) or bro-mocriptine. These findings imply pathology of extrastriatal dopamine systems in Parkinson disease. Since the addition of cilrbidopa enhanced the suppression of prolactin induced by levodopa, exogenous levodopa probably acts predominantly through the formation of dopamine in the hypothalamus, but inside the blood-brain barrier, rather than as a direct effect of circulating dopamine on the anterior pituitary or arms of the hypothalamus outside the blood-brain barrier.

Phorbol esters affect pituitary growth hormone (GH) and prolactin release: The interaction with GH releasing factor, somatostatin and bromocriptine

Phorbol esters are tumor promotors that directly stimulate protein kinase C activity. We asked whether these agents affect basal or receptor initiated alterations in growth hormone (GH) and prolactin release. In 4 h incubations of anterior pituitary cells, phorbol esters enhanced basal and GH releasing factor (GRF)-induced GH release. Somatostatin reduced by 38% the 4-fold stimulation of GH release induced by phorbol ester. These tumor promoters also reversed the ability of bromocriptine, a dopamine agonist, to inhibit prolactin release, with no apparent effect on basal prolactin secretion. When these agents were applied for 24 h, an increase in the basal release of both GH and prolactin was apparent. These data lead us to suggest that an intact protein kinase C system may be necessary for the full expression of GRF-stimulated GH release and dopaminergic inhibition of prolactin release.

Activation of calcium channels by Maitotoxin

Publisher Summary This chapter describes three techniques that enhance the understanding of the action of maitotoxin by using 45Ca2+ to monitor calcium ion flux. The techniques are not new but are adapted for use with viable dispersed anterior pituitary cells in suspension. Methodology for monitoring 45Ca2+ uptake, fractional 45Ca2+ efflux, and net 45Ca2+ efflux are described with reference to the effects of maitotoxin on these processes. Maitotoxin is a water-soluble material extracted from a marine dinoflagellate and bears the Tahitian name for a species of surgeonfish in which it was also recognized. Maitotoxin is not yet characterized but is extremely lethal when injected into mice; it stimulates a wide variety of physiological processes, all sharing the apparent property of being dependent on extracellular calcium. The spectrum of its activity and the apparent lack of ionophoretic properties suggest that maitotoxin activates calcium channels to exert its biological actions.