Adenosine Inhibits Prolactin and Growth Hormone Secretion in a Clonal Pituitary Cell Line*

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Abstract
Although purine nucleosides have been shown to regulate the secretion of several peptide and steroid hormones, effects on pituitary hormone release have not been reported. We have shown here that in the clonal GH4C1 pituitary cell line maximal concentrations of adenosine (.gtoreq. 50 .mu.M) inhibited PRL and GH secretion by 40%. Adenosine deaminase abolished the inhibitory affect of adenosine but not that of SRIF or (-)N6(R-2-phenylisopropyl)adenosine (PIA), a nonhydrolyzable adenosine analog. Furthermore, this enzyme increased basal secretion by 50%, and analysis of the incubation medium by HPLC demonstrated that the cells secreted biologically effective concentrations of adenosine. These results indicate that adenosine produced in culture tonically inhibits hormone release. In other target cells, adenosine inhibition is mediated by 2 types of binding sites: an extracellular Ri-site requiring an intact ribose moiety or an intracellular P-site requiring an intact purine ring. Four lines of evidence indicate that in GH4C1 cells, adenosine acts at an Ri-site. (1) PIA, an Ri-site-specific agonist, was a potent inhibitor of hormone release (ED50 = 30 nM). (2) Theophylline, an Ri-site antagonist, competitively inhibited the action of PIA (Ki = 2.4 .mu.M). (3) 2''5''-Dideoxyadenosine, a P-site-specific agonist, did not inhibit PRL release even at a concentration of 1 mM. (4) Dipyridamole, an adenosine uptake inhibitor, did not reduce adenosine inhibition. In addition to its effect on basal secretion, PIA inhibited stimulation of hormone release by vasoactive intestinal peptide and TRH. PIA also reduced vasoactive intestinal peptide-stimulated cAMP accumulation by 75%, consistent with its action to inhibit adenylate cyclase via Ri receptors in other targets. Since PIA inhibition of PRL release and cAMP accumulation was not additive with the effects of SRIF and carbamyl choline, these inhibitors may act via a common rate-limiting step. Our results demonstrate that adenosine activates an Ri-type of adenosine receptor in GH4C1 cells and that the production of adenosine under normal culture conditions causes autocrine inhibition of secretion.