Muscarinic effects of vagosympathetic trunk stimulation on the repetitive extrasystole (RE) threshold

The precise mechanism of the vagus in opposing adrenergic influences on cardiac vulnerability has not been defined. In the present investigation, cholinergic stimulation was produced by administering the selective muscarinic agent methacholine (MCh) and ventricular vulnerability was assessed by measuring the repetitive extrasystole (RE) threshold. MCh produced a sustained 98 +/- 12% (P less than 0.0001) increase in the RE threshold; the increase was abolished by concurrent infusion of atropine (0.2 mg/kg). When MCh was administered during beta-adrenergic blockade with propranolol, however, no further increase in RE threshold occurred beyond that resulting from propranolol alone. Infusion of norepinephrine (NE), in a dose insufficient to provoke systemic hypertension, significantly lowered the vulnerable period threshold. This decrease was completely abolished by concurrent vagosympathetic stimulation (VS). It is concluded that VS affects ventricular vulnerability through its muscarinic action and that the protection against VF is in part due to modulation of neural and humoral beta-adrenergic inputs.