title: Role of calcium and sodium ions in the inhibitory control of baseline and stimulated prolactin release.
abstract: The mechanism of dopamine (DA) inhibition of pituitary PRL release is still unclear. To study it, we utilized enzymatically dispersed anterior pituitary cells obtained from adult female Sprague-Dawley rats. The cells were incubated in media with or without Na+ and in the presence or the absence of various drugs for 30 min for evaluating the secretion of PRL under baseline and experimental conditions. In some experiments, 45Ca2+ (1 microCi/ml) was added after 30 min of incubation and the latter prolonged for an additional minute to determine Ca2+ uptake. DA inhibited baseline PRL release and 45Ca2+ uptake in a dose-dependent manner only in the presence of Na+ and was totally inactive in its absence. The inhibitory effects of Nifedipine and Nicardipine, two Ca2+ channel antagonists, on PRL release were also found to be Na+ dependent. BAY K 8644, a Ca2+ channel agonist, stimulated PRL release and Ca2+ uptake in a dose-dependent manner, and these effects were enhanced by Na+-free media. DA antagonized the stimulatory actions of BAY K 8644 on PRL release in a similar dose-dependent manner both in the presence and the absence of Na+. However, on stimulated 45Ca2+ uptake DA was less effective in the absence of Na+. The stimulatory action of TRH on PRL release was enhanced by the absence of Na+. DA antagonized the effect of TRH in a dose-dependent manner both in the presence and in the absence of Na+ but appeared more effective in the absence of the ion. The PRL-releasing effects of phorbol ester and of the Ca2+ ionophore A23187 were antagonized by DA in a Na+- independent manner. These results suggest the existence of two mechanisms of DA inhibitory action: one exerted on baseline PRL release which is Na+ dependent, receptor linked, and probably implicates potential operated Ca2+ channels; the other is exerted on stimulated PRL release, is Na+ independent, and appears to be a postreceptorial intracellular event probably involving protein kinase C and/or cytosolic Ca2+ levels.
mesh_terms: D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D004298:Dopamine / Q000494:pharmacology; D005260:Female; D009529:Nicardipine / Q000494:pharmacology; D009543:Nifedipine / Q000494:pharmacology; D010703:Phorbol Esters / Q000494:pharmacology; D011388:Prolactin / Q000097:blood*; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000502:physiology*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology
chemical_list: D010703:Phorbol Esters; D000001:Calcimycin; D013973:Thyrotropin-Releasing Hormone; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D011388:Prolactin; D012964:Sodium; D009529:Nicardipine; D009543:Nifedipine; D002118:Calcium; D004298:Dopamine