Effect of clonidine on cardiac baroreflex delay in humans and rats

The delay τ between rising systolic blood pressure (SBP) and baroreflex bradycardia has been found to increase when vagal tone is low. The α2-agonist clonidine increases cardiac vagal tone, and this study tested how it affects τ. In eight conscious supine human volunteers clonidine (6 μg/kg po) reduced τ, assessed both by cross correlation baroreflex sensitivity and sequence methods (both P < 0.05). Experiments on urethane-anaesthetized rats reproduced the phenomenon and investigated the underlying mechanism. Heart rate (HR) responses to increasing SBP produced with an arterial balloon catheter showed reduced τ (P < 0.05) after clonidine (100 μg/kg iv). The central latency of the reflex was unaltered, however, as shown by the unchanged timing with which antidromically identified cardiac vagal motoneurons (CVM) responded to the arterial pulse. Testing the latency of the HR response to brief electrical stimuli to the right vagus showed that this was also unchanged by clonidine. Nevertheless, vagal stimuli d...