Dopamine Inhibits the Arachidonate and Prolactin Release Stimulated by Thyrotropin-Releasing Hormone through an Islet-Activating Protein-Sensitive GTP-Binding Protein in Anterior Pituitary Cells

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Abstract
Coupling of the dopamine (DA) receptor to the pathway of arachidonate (AA) through the islet-activating protein (IAP)-sensitive GTP-binding (G) protein was examined in primary cultures of anterior pituitary cells. The inhibitions by 1 µM DA of the stimulations of prolactin (PRL) release by 100 nM thyrotropin-releasing hormone and by 10 µM calcium ionophore A23187 were blocked by 100 ng/ml IAP. DA at 1 µM did not inhibit the release of [3H]-AA induced by 100 mU/ml phospholipase A2 (PLA2), but it inhibited the release of PRL induced by 100 mU/ml PLA2. This inhibition was blocked by 100 ng/ml IAP. IAP also blocked the inhibitions by DA of the releases of PRL by 5 µM AA and by 5 µM 5-hydroxyeicosatetraenoic acid. These results suggest that the DA receptor on lactotrophs is coupled to the pathway following the release of AA (lipoxygenase) through the IAP-sensitive G protein.