Pertussis toxin treatment blocks hyperpolarization by muscarinic agonists in chick atrium.

Abstract

Atrial and ventricular adenylate cyclase activity and atrial membrane potentials were measured in hearts from hatched chicks at 2-3 days after intravenous administration of pertussis toxin (0.5-1.0 micrograms, total) or saline. Both in atrium and ventricle, treatment with pertussis toxin antagonized inhibition by carbachol of basal and isoproterenol-stimulated adenylate cyclase activity without changing either basal or isoproterenol-stimulated adenylate cyclase. In atria from pertussis toxin-treated animals (5.4 mM potassium), carbachol hyperpolarized the resting membrane by 0.3 +/- 0.3 mV (n = 9) and did not increase resting potassium conductance. In contrast, carbachol hyperpolarized the resting membrane by 4.5 +/- 0.8 mV (n = 11) and increased resting potassium conductance more than 4-fold in saline-treated animals. Carbachol did not significantly affect the atrial action potential peak or duration at 50% repolarization of pertussis toxin-treated animals. This muscarinic agonist reduced action potentia...