Inhibition of neuronal nitric oxide reduces heart rate variability in the anaesthetised dog

In the vagally intact anaesthetised dog, we have investigated the role of n itric oxide (NO) on a normal sinus arrhythmia using an inhibitor of neurona lly released NO, 1-(2-trifluoromethylphenyl) imidazole (TRIM). The mean and S.D. of the R-R interval was used to describe mean heart rate and heart ra te variability, respectively. TRIM (0.8 mg I.C.) injected into the sinus no de artery increased the mean heart rate slightly but reduced heart rate var iability 3-fold from a control of 790 +/- 124 ms (mean S.D.; n = 5) to 666 +/- 36 ms (P < 0.01 Student's paired t test, n = 5). These results suggest that neuronally released NO may have a vagal facilitatory role in the maint enance of sinus arrhythmia in the normal heart.