Cardiac vagal effects of rilmenidine in the dog: comparison with clonidine

1. The cardiac vagal effects of rilmenidine (5 micrograms kg-1 min-1) and clonidine (0.5 micrograms kg-1 min-1) were studied in chloralose anaesthetized dogs. 2. Rilmenidine and clonidine progressively reduced the vagal stimulation-induced bradycardia. As indicated by the ED70, rilmenidine was about 23 times less potent than clonidine in this respect. Concomitantly, both drugs dose-relatedly decreased heart rate and mean blood pressure with potency ratios of rilmenidne to clonidine of about 1:23 and 1:12, respectively. 3. Importantly, the heart rate values observed under vagal stimulation during drug infusion never exceeded the values under basal vagal stimulation, and with both drugs large interindividual variations occurred under vagal stimulation. 4. These results show that the vagal bradycardia inhibition produced by rilmenidine and clonidine results from their true bradycardic effects and not from actual cardiac vagolytic properties.