Inhibition of nitric oxide synthase slows heart rate recovery from cholinergic activation

University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, United Kingdom The role of nitric oxide (NO) in the cholinergic regulation of heart rate (HR) recovery from an aspect of simulated exercise was investigated in atria isolated from guinea pig to test the hypothesis that NO may be involved in the cholinergic antagonism of the positive chronotropic response to adrenergic stimulation. Inhibition of NO synthesis with N G -monomethyl- L -arginine ( L -NMMA, 100 µM) significantly slowed the time course of the reduction in HR without affecting the magnitude of the response elicited by bath-applied ACh (100 nM) or vagal nerve stimulation (2 Hz). The half-times ( t 1/2 ) of responses were 3.99 ± 0.41 s in control vs. 7.49 ± 0.68 s in L -NMMA ( P