Hyperpolarization is not responsible for the acetylcholine-induced negative chronotropic action in the presence of isoproterenol.

Abstract

It has been reported that acetylcholine hyperpolarizes the maximum diastolic potential of canine Purkinje fibers through a pathway involving a G protein and induces a decrease in their automaticity. It is unclear, however, whether the negative chronotropic action of acetylcholine in the presence of beta-adrenergic stimulation is due to the hyperpolarization of the maximum diastolic potential or a decrease in the slope of phase 4 depolarization. We used standard microelectrode techniques to study the negative chronotropic mechanism of acetylcholine in the presence of isoproterenol in adult canine Purkinje fibers. Fibers were incubated for 24 hours in Tyrode's solution alone (n = 10) or plus pertussis toxin (n = 10), and then superfused with acetylcholine (10(-9) to 10(-4) M) in the presence of isoproterenol (10(-7) M). Acetylcholine in the presence of isoproterenol significantly decreased automaticity without hyperpolarization of the maximum diastolic potential, and decreased the slope of phase 4 depolarization. The effects of acetylcholine on automaticity and the slope of phase 4 depolarization were attenuated by pertussis toxin. The present findings indicate that the negative chronotropic action of acetylcholine in the presence of isoproterenol is due to the decrease in the slope of phase 4 depolarization through a pathway involving a pertussis toxin-sensitive G protein and that it is not the result of hyperpolarization of the maximum diastolic potential.