Attenuation of Anterior Pituitary Phosphoinositide Phosphorylase Activity by the D2Dopamine Receptor*

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Abstract
We have examined the influences of dopamine and the D2 receptor agonist bromocriptine on phosphoinositide metabolism in primary cultures of rat anterior pituitary cells, monitoring changes in the levels of phosphatidylinositol (PtdIns), phosphatidylinositol-4-phosphate [PtdIns(4)P], and phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P]2. Basal incorporation of [3H]inositol ([3H]Ins) into phosphoinositides was progressive, and radioisotopic equilibrium was attained in all three species within 48 h. The inclusion of dopamine or bromocriptine in the incubation medium promoted concentration-dependent reductions in the rate, but not the magnitude, of phosphoinositide radiolabeling. The onset of this effect was rapid; inhibition of [3H]Ins incorporation by dopamine (500 nM) and bromocriptine (100 nM) could be detected within 2 h. This treatment also produced a comparable reduction in the incorporation of [32P]orthophosphate into PtdIns(4,5)P2. In extended time-course studies, bromocriptine dramatically retarded the radiolabeling of PtdIns(4)P and PtdIns(4,5)P2, and apparent equilibria in these species were attained only after 96 h. We also assessed the ability of dopamine to modify the concentration-response characteristics of [3H]Ins-labeled inositol phosphate ([3H]InsPx) production by TRH, angiotensin II (AII), neurotensin (NTS), bombesin (BBS), and vasoactive intestinal polypeptide (VIP). Neither dopamine nor bromocriptine altered the rate or magnitude of TRH-, AII-, NTS-, or BBS-related InsPx generation. VIP was completely ineffective in stimulating InsPx generation. PRL release was significantly reduced in all dopamine-treated groups. That the InsPx concentration-response relationships for each of these peptides remained unimpairied by exposure to dopamine or bromocriptine extends our previous observation that the phosphoinositide-specific phopholipase-C is insensitive to dopaminergic tone. Consistent with our earlier findings, these data indicate that activation of the D2 dopamine receptor attenuates the activity of mechanisms associated with the serial phosphorylations of PtdIns and PtdIns(4)P, reactions that give rise to PtdIns(4)P and PtdIns(4,5)P2, respectively. It is our conclusion that dopamine, in addition to its other actions, attenuates the phosphorylation, rather than the hydrolysis, of anterior pituitary phosphoinositide. This attenuation appears to be mediated by an inhibitory coupling of the D2 receptor with the phosphoryltransferase activities that catalyze PtdIns(4)P and PtdIns(4,5)P2 formation. In parallel studies, we also determined whether such a mechanism exists for somatostatinergic regulation of phosphoinositide metabolism in the anterior pituitary. Both somatostatin and the long-acting analog SMS 201-995 produced substantial inhibitions of GH and PRL release. However, such treatment had no effect on the formation of [3H]PtdIns, [3H]PtdIns(4)P, and [3H]PtdIns(4,5)P2 or, similarly, on the generation of [3H]InsPx elicited by TRH, AII, NTS, or BBS. Neither the phosphorylation nor the hydrolysis of anterior pituitary phosphoinositide, therefore, appears to be sensitive to somatostatin receptor activity. In view of these data, we propose that the capacity to reduce phosphoinositide phosphorylation may be a unique property of the D2 dopamine receptor, representing an aspect of the chronic mechanism by which dopamine inhibits PRL release.

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