THERE is evidence for the existence of multiple forms of dopamine (DA) receptors1-3. In particular, some of these receptors are coupled to adenylate cyclase (DA-stimulating enzyme activity), whereas others seem to be independent3-5. These two classes of receptors, which have been designated D-1 and D-2, respectively, would also have different pharmacological properties3. Pituitary mammotroph DA receptors regulating prolactin (PRL) secretion are considered to be typical D-2 receptors. A DA-stimulated adenylate cyclase has not been detected in normal anterior pituitaries 3,5,6; furthermore, several studies indicate that cyclic AMP is stimulatory and not inhibitory to PRL secretion7,8. We report here that in homogenates of human PRL adenomas, in which dopaminergic agonists act as inhibitors of PRL secretion, basal adenylate cyclase is inhibited by DA. The DA receptors mediating this inhibition have the same pharmacological properties as those regulating PRL secretion.
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