INTERACTION BETWEEN ADRENERGIC AGONISTS AND FORSKOLIN ON ADENYLATE-CYCLASE ACTIVITY IN THE RABBIT PROXIMAL TUBULE

Using a micro-radioimmunoassay, cAMP was measured in single, isolated S2 proximal straight tubules dissected from rabbit kidneys to investig ate the effects of adrenergic agonists on adenylate cyclase activity. The baseline activity of adenylate cyclase was low and unaffected by e ither the alpha(2) agonist clonidine, the beta agonist isoprenaline (i n the absence or presence of 1 mu M forskolin) or 1 mu M forskolin. Ad enylate cyclase activity was markedly stimulated by 20 mu M forskolin, an effect which was inhibited by 1 mu M clonidine. The inhibition by clonidine was not apparent in the presence of the alpha(2) antagonist yohimbine. These results confirm the inability of isoprenaline to stim ulate adenylate cyclase in the rabbit proximal tubule and demonstrate the coupling of alpha(2) receptors, in an inhibitory fashion, to adeny late cyclase. The inhibitory action of the alpha(2)-receptor agonist w as independent of other hormone activity in the renal proximal tubule.