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.