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 investigate the effects of adrenergic agonists on adenylate cyclase activity. The baseline activity of adenylate cyclase was low and unaffected by either the α2 agonist clonidine, the β agonist isoprenaline (in the absence or presence of 1 μM forskolin) or 1 μM forskolin. Adenylate cyclase activity was markedly stimulated by 20 μM forskolin, an effect which was inhibited by 1 μM clonidine. The inhibition by clonidine was not apparent in the presence of the α2 antagonist yohimbine. These results confirm the inability of isoprenaline to stimulate adenylate cyclase in the rabbit proximal tubule and demonstrate the coupling of at receptors, in an inhibitory fashion, to adenylate cyclase. The inhibitory action of the α2-receptor agonist was independent of other hormone activity in the renal proximal tubule.