Developmental changes in the muscarinic stimulation of canine Purkinje fibers.

Acetylcholine (ACh) hyperpolarizes adult canine Purkinje fibers and induces a decrease in their automaticity. In Purkinje fibers from young dogs, there is a biphasic effect on automaticity, which increases at low and decreases at high ACh concentrations. We used standard microelectrode techniques to study these actions of ACh. In fibers from young dogs, 10(-10) to 10(-9) M ACh increased automaticity and 10(-5) M ACh decreased automaticity. The decrease was blocked by the M2 muscarinic blocker AFDX-116, whereas the increase was blocked by the predominant M1 blocker pirenzepine. The M2 agonist oxotremorine never increased automaticity. Rather, it decreased automaticity and hyperpolarized adult and young fibers, the former more than the latter. The hyperpolarization and biphasic effect on automaticity of ACh in fibers from young dogs failed to occur after treatment with pertussis toxin, suggesting that these effects are dependent on a pertussis toxin-sensitive G protein. These electrophysiologic studies suggest that postsynaptic M1 and M2 muscarinic processes modulate the automatic response of Purkinje fibers from young dogs and that the postsynaptic M1 pathway is no longer seen in the adult.