Purification and cloning of brain isoform of ADP-ribosyl cyclase

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We examined the role of cyclic ADP-ribose (cADP-ribose) as a second messenger downstream of adrenergic receptors in the heart after excitation of sympathetic neurons. To address this question, ADP-ribosyl cyclase activity was measured as the rate of [3H]cADP-ribose formation from [3H]NAD+ in a crude membrane fraction of rat ventricular myocytes. Isoproterenol at 1 mM increased ADP-ribosyl cyclase activity by 1.7-fold in venticular muscle ; This increase was inhibited by propranolol. The stimulatory effect on the cyclase was mimicked by 10 nM GTP and 10 mM GTP-g-S, while 10 mM GTP inhibited the cyclase. Cholera toxin blocked the activation of the cyclase by isoproterenol and GTP. The above effects of isoproterenol and GTP in ventricular membranes were confirmed by cyclic GDP-ribose formation fluorometrically. These results demonstrate the existence of a signal pathway from b-adrenergic receptors to membrane-bound ADP-ribosyl cyclase via G protein in the ventricular muscle cells, and suggest that increased cADP-ribose synthesis is involved in upregulation of cardiac function by sympathetic stimulation.