Differential effect of pertussis toxin on adenosine and muscarinic inhibition of cyclic AMP accumulation in canine ventricular myocytes.

Cyclic AMP regulation by muscarinic and adenosine receptors was investigated in isolated canine ventricular myocytes. Both the muscarinic receptor agonist, carbachol, and the adenosine receptor agonist, phenylisopropyladenosine, decreased isoproterenol-stimulated cyclic AMP accumulation in a concentration-dependent manner. Carbachol was more potent than phenylisopropyladenosine and had a greater inhibitory effect. At 10(-6) M, carbachol reduced isoproterenol-stimulated cyclic AMP by 73 +/- 5% while 10(-3) M phenylisopropyladenosine was required to decrease cyclic AMP accumulation by 54 +/- 8%. Pretreatment of myocytes with pertussis toxin to inactivate the inhibitory guanine nucleotide binding protein, Gi, completely abolished the effect of phenylisopropyladenosine to reduce cyclic AMP stimulation. In comparison, pertussis toxin treatment blunted the response to carbachol and shifted the dose-effect curve to the right but did not eliminate the inhibitory action of carbachol. In toxin-treated myocytes, 10(-3) M carbachol produced a 26 +/- 6% reduction of isoproterenol-induced cyclic AMP accumulation. This pertussis toxin-insensitive action of carbachol was antagonized by atropine and pirenzepine and was prevented when either of two different phosphodiesterase inhibitors. RO-20-1724 or isobutylmethylxanthine, was included in the incubation medium. The results indicate that adenosine receptor-mediated inhibition of hormone-stimulated cyclic AMP accumulation in ventricular myocytes occurs by a single, Gi-dependent mechanism while muscarinic inhibition appears to involve both Gi-dependent and Gi-independent mechanisms. The Gi-independent mechanism may reflect enhanced phosphodiesterase activity which results from the activation of muscarinic receptors.