CATECHOLAMINERGIC CONTROL OF INTRACELLULAR FREE CALCIUM AND BETA-ENDORPHIN SECRETION OF RAT PITUITARY INTERMEDIATE LOBE CELLS

Individual melanotropes and intermediate lobes were tested to elucidat e the role of alpha- and beta-adrenergic and D-2 dopamine receptors in the regulation of concentration of intracellular free calcium ([Ca2+] (i)) and release of beta-endorphin. Hormone secretion was studied in a superfusion system, while [Ca2+](i) was measured microspectrofluorime trically. Noradrenaline (1 mu M) resulted in a slight decrease, then a marked increase in [Ca2+](i) and secretion of beta-endorphin. The non selective beta-adrenergic agonist isoproterenol (1 mu M) increased [Ca 2+](i) and secretion of beta-endorphin; this effect was blocked by the beta-antagonist propranolol (10 mu M). The alpha(1)-adrenergic agonis t phenylephrine (1 mu M) increased [Ca2+](i) and beta-endorphin secret ion, but this effect was not blocked by terazosin or prazosin (alpha(1 )-adrenergic antagonists, 1 mu M). Administration of the alpha(2)-adre nergic agonist xylazine (I mu M) increased [Ca2+](i) but did not affec t secretion of the hormone, Biphasic effect of noradrenaline was teste d in presence of adrenergic and dopaminergic antagonists, The noradren aline-induced rise in [Ca2+](i) and beta-endorphin secretion was decre ased by propranolol, but this drug did not modify the inhibition, In t he presence of 1 mu M sulpiride (selective D-2 dopaminergic antagonist ), the inhibitory phase of the curve was abolished, and the subsequent increase was reduced. This suggests that activation of dopamine D-2 r eceptors is involved not only in the inhibition, but also in the subse quent increase, which may originate from a rebound after the terminati on of the activation of these inhibitory receptors. Our data suggest t he presence of several distinct types of catecholamine receptors in th e rat intermediate robe, and the dominant involvement of D-2 and beta- adrenergic receptors in the noradrenaline-induced regulation of melano tropes.