MMQ CELLS - A MODEL FOR EVALUATING THE ROLE OF G-PROTEINS IN THE MODULATION OF PROLACTIN-RELEASE

It is well known that dopamine (DA) inhibits while vasoactive intestinal peptide (VIP) and angiotensin II (ANG II) stimulate prolactin (PRL) release from normal anterior pituitary lactotrophs; however, elucidation of the intracellular mechanisms involved in these effects has been hindered by the cellular heterogeneity of the anterior pituitary. MMQ cells, isolated from the PRL-secreting rat pituitary tumor 7315a is an interesting model since they only secrete PRL. In order to determine whether and which GTP-binding (G) proteins are involved in the modulation of cyclic 3',5'-adenosine monophosphate (cAMP) accumulation and phospholipids turnover and eventually PRL release, we have performed studies with MMQ cells. For this purpose, the levels of various G proteins (alphao, alphas, alphai, alphaq and beta) and their mRNAs, measured by Western and Northern blots respectively, were correlated with intracellular cAMP accumulation in response to DA, VIP or DA plus VIP, and with inositol phosphates (IPx) formation in response to ANG II, DA or DA plus ANG II. This study shows that, when compared to normal pituitary tissue, the levels of alphao, alphao2 and alphai3 were significantly decreased in MMQ cells; those of alphao1, alphai (alphai1 + alphai2), alphas42 and alphaq were very low or undetectable while those of alphas47 and beta were normal. DA was unable to inhibit basal PRL release and cAMP accumulation. VIP increased both cAMP accumulation and PRL release, while cAMP accumulation elicited by VIP could be suppressed by DA. BAY K 8644-induced PRL release also could be suppressed by DA. DA and, paradoxically, ANG II decreased IPx formation; the action of ANG II could be blocked by SAR,ILE-ANG II and was exerted through pertussis toxin-sensitive G proteins. This study and previously published results provide circumstantial evidence that alphao, alphai1 and alphai2 are all involved in the transduction of the DA inhibitory message while alphas47 transduces cAMP activating messages and alphas42 is responsible for the constitutive activation of L-type Ca2+ channels, adenylate cyclase and baseline PRL release.