Abstract
In primary culture of anterior pituitary cells, dopamine inhibited the angiotensin (AII)-stimulated inositol phosphate production by 28 +/- 2.5% (n = 14), with an EC50 of 660 +/- 228 nM (n = 8). This effect was blocked by (+)-butaclamol, a specific dopamine receptor antagonist. RU 24926, a D2 specific agonist, but not SKF 38393, a specific D1 agonist, inhibited AII-stimulated inositol phosphate production, suggesting that this dopamine effect is mediated through a dopamine receptor of the D2 subtype. Dopamine also partially inhibited (25%) inositol phosphate production stimulated by thyrotropin-releasing hormone (TRH). Our results suggest that the dopamine-mediated inhibition of hormonally stimulated inositol phosphate production is probably not mediated through the known inhibitory effects of dopamine on cAMP and Ca2+ intracellular concentrations. Although unknown, the mechanism by which dopamine inhibited the AII and TRH-stimulated inositol phosphate production implicates a GTP binding protein sensitive to the islet activating protein (IAP) since dopamine effects were blocked by this toxin. The alpha subunit of the GTP binding protein involved could be one of the three ADP-ribosylated proteins found in anterior pituitary cells in primary cultures, the alpha o (39 kDa), the alpha i (41 kDa), and an alpha subunit of 40 kDa. Indeed, we show here that this 40-kDa IAP substrate, already described in a few tissues, is present in anterior pituitary cells. The negative coupling between dopamine receptors and the AII or TRH inositol phosphate production systems, could be implicated in the dopamine inhibition of the AII- and TRH-stimulated prolactin release since such an inhibition is blocked by IAP. Our results suggest that the negative regulation of inositol phosphate production is one of the mechanisms by which dopamine controls hormonally stimulated prolactin release.
Highlights
In primary cultureof anterior pituitarycells, dopa- agents increasing either cAMP (for example, vasoactive inmine inhibited the angiotensin (AI1)-stimulated inosi- testinal peptide, forskolin, 8-bromo-CAMP) [7,8,9,10] or intratol phosphate production by 28 f 2.5% (n= 14),with an EC,of 660 f 228 nM (n = 8)
thyrotropin-releasing hormone (TRH) inositol phosphate production systems, could be implicated in the dopamine inhibition of the AII- and TRH-stimulated prolactin release since such an inhibition is blocked by IAP
Our results suggest that the negative regulationof inositol phosphate productionis one of the mechanisms by which dopamine controls finding andshow that thisnew dopaminergic effect occurs on both AII- and TRH-stimulated inositol phosphate production, that it is mediated through a D, dopaminergic receptor, is blocked by pertussis toxin, and islikely to be independent to the decrease in cyclic AMP productioninduced by dopamine
Summary
Effects of Dopamine (DZ)Receptor on Basal and AII-stimulated Inositol Phosphate Productioannd Prolactin Secretion-. AI1 (100 nM) stimulatedinositolphosphateproductionin anterior pituitary cells in primary cultures by3.2 f 0.18-fold ( n = 16) (Fig. 1A) Dopamine inhibited this stimulated inositol phosphate production ina dose-dependent manner (Fig. 1A) with an EC,, of 660 k 228 nM ( n = 8) a n d a maximal effect of 18 f 1.6% ( n = 14). 1. Dose-dependent effect of dopamine (DA) on basal and AII-stimulated inositol phosphate production and prolactin secretion. (+)-butaclamol (10 p ~ ) a, potent dopaminergic antagonist, completely blocked the dopamine dose-dependent inhibition of AII-stimulated inositol phosphate productionR. Secretion-A 24-h pretreatment of anterior pituitary cells in primary culture with 200 ng/ml IAP completely suppressed the dose-dependent dopamine blockadofe AII-stimulated inositol phosphate formation (Fig.3A). Stimulated inositol phosphate production but completely reversed the dopamine inhibition of TRH effect (Fig. 0 ) .In the same experiment the dopamine inhibition of T R H - i n - ducedprolactinsecretionwascompletely blocked by IAP pretreatment
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