Adenosine-sensitive adenylate cyclase in rat anterior pituitary.

An adenosine-sensitive adenylate cyclase has been demonstrated in anterior pituitary cultured cells in the present studies. N-ethylcarboxamideadenosine (NECA), L-N6-phenylisopropyladenosine (PIA), and 5'-N-methylcarboxamideadenosine (MECA), all stimulated adenylate cyclase in a concentration-dependent manner in the order of potency NECA greater than PIA greater than MECA. Adenosine showed a biphasic effect on adenylate cyclase: stimulation at lower and inhibition at higher concentrations, whereas 2'-deoxyadenosine only inhibited adenylate cyclase in a concentration-dependent manner. The stimulatory effect of NECA on adenylate cyclase was dependent on metal ion concentrations and was blocked by 3-isobutyl-1-methylxanthine and 8-phenyltheophylline. Various agonists such as isoproterenol, prostaglandins (PGE1), vasoactive intestinal peptide, corticotropin-releasing factor, NaF, and forskolin, all stimulated adenylate cyclase to various degrees. The stimulatory effect of vasoactive intestinal peptide and corticotropin-releasing factor on adenylate cyclase was found to be almost additive with the stimulation exerted by NECA. These data indicate the presence of adenosine stimulatory receptors ('Ra') in anterior pituitary which are coupled to adenylate cyclase. It is possible that adenosine may act as one of the important regulators to regulate and/or modulate the effects of agents/factors in the release of pituitary hormones.