Studies on Receptor-mediated Activation of Adenylyl Cyclases

The characteristics of the response of the beef renal medullary adenylyl cyclase system to prostaglandins (PG) were studied. This system is stimulated in decreasing order of efficacy, by PGE1, PGE2, and PGA1; PGF1α and PGF2α are ineffective. Optimal effects of the active PGs are obtained at low Mg2+ concentrations (0.1 to 0.5 mm) and in the presence of micromolar concentrations of GTP or millimolar concentrations of ATP. Neither PGE1 nor PGE2α interfere either in the absence or in the presence of 10-5 m GTP with the action of half-maximally or maximally stimulating concentrations of neurohypophyseal hormones (NHH). Further studies centered on the characterization of patterns of modulation by purine nucleotides of prostaglandin and hormonal stimulation of adenylyl cyclases in other tissues and comparison to the pattern observed in renal medulla. Using 5'-adenylyl-imidodiphosphate as substrate the effects of nucleoside diphosphates and of nucleoside triphosphates were studied respectively in the absence and in the presence of nucleotide triphosphate regenerating system. It was found that prostaglandin response in renal medullary membranes is unaffected by GDP, ADP, ITP, and IDP. In contrast, prostaglandin response in membrane particles from corpora lutea of pregnant rabbits is enhanced by GDP and unaffected by GTP and adenosine and inosine di- and triphosphates. Hormonal stimulation in these two tissues is regulated differently than prostaglandin stimulation. Stimulation of corpus luteum adenylyl cyclase by luteinizing hormone requires the presence of 1 to 3 mm ATP and is inhibited by ADP and, to a lesser extent, by IDP; guanylyl nucleotides and ITP are without effect on stimulation by luteinizing hormone. Stimulation of renal medullary membrane adenylyl cyclase by NHH, on the other hand, is enhanced by micromolar concentrations of GDP and inhibited by micromolar concentrations of GTP and millimolar concentrations of ATP and ITP; ADP and IDP are without effect. Finally, response of cat ventricular adenylyl cyclase to catecholamines was found to be enhanced by GTP and inhibited by GDP, effects opposite to those seen for NHH stimulation in renal medullary membranes. These findings indicate the existence of several modes of nucleotide regulation. These modes vary with the tissue studied and, independently, with the stimulatory agent. It is concluded that hormonal response of cells is not simply a passive receptor-dependent phenomenon and that the interaction between adenylyl cyclase and occupied receptor(s) is susceptible to regulation by metabolism of the target cell, be it normal or modified by other hormones or disease. Since GTP was also found to increase adenylyl cyclase activities in corpus luteum membrane particles without affecting the ratio of stimulation by either prostaglandins (whose action is enhanced by GDP) or luteinizing hormone (whose action is enhanced by ATP), and since GTP was found in other systems also to affect the binding of the stimulating hormone to receptors coupled to adenylyl cyclase (Rodbell, M., Lin, M. C., and Salomon, Y. (1974) J. Biol. Chem. 249, 59–65; Nakahara, T., and Birnbaumer, L. (1974) J. Biol. Chem. 249, 7886–7891) it is suggested that the different effects of nucleotides are the result of their interaction with distinct sites of hormone sensitive adenylyl cyclase systems, each responsible for a separate effect, be it modification of adenylyl cyclase activity, modification of hormonal stimulation or modification of rate (or rates) of receptor-hormone interaction.