Modulation of Vagal Actions on Heart Rate Produced by Inhibition of Nitric Oxide Synthase in the Anaesthetized Ferret

We have examined a possible role for nitric oxide in the efferent vagal control of heart rate of the anaesthetized ferret, using stimulation of the motor fibres of the right vagus at supramaximal intensities and increasing frequencies. Propranolol was used to block beta-adrenergic actions on the heart. L-NAME (50-100 mg i.v.) significantly reduced the bradycardia induced by vagal stimulation at all frequencies tested (1-15 Hz); subsequent infusion of L-arginine (100-200 mg i.v.) restored the vagally induced heart rate responses to control levels. These results strongly suggest a role for nitric oxide in modulating the vagally mediated control of heart rate.