Fentanyl antagonizes diazepam on carotid sinus baroreflex control of circulation in rabbits.

To investigate the effects of a combination of fentanyl and diazepam on carotid sinus baroreflex in conscious rabbits, we examined the responses of mean systemic arterial pressure (MAP), heart rate (HR) and total peripheral resistance (TPR) to bilateral carotid occlusion (BCO). Seven rabbits were given 0.5 mg.kg(-1) of diazepam i.v. followed by 10 mg.kg(-1) of fentanyl i.v. at 5 min intervals (group 1), and the drugs were given in the reverse order to 5 other rabbits (group 2). BCO was repeated in conscious state (control) and after each drug injection. MAP responses did not differ from control response in either group when both drugs were given. In group 1, however, diazepam decreased HR response to 71.4% of control, and increased TPR response by 36%. Fentanyl administration reversed diazepam-induced changes in BCO responses to the control level. In group 2, fentanyl decreased TPR response to 61.6% of control and increased HR response by 41.5%. Administration of diazepam following fentanyl restored HR and TPR responses to control levels. Carotid sinus baroreflex gain was 3.1 +/- 0.4 (mean +/- SEM) in control and 3.1 +/- 0.4 after administration of both drugs in 12 rabbits. The results suggest that a sedative dose of either fentanyl or diazepam antagonizes the other drug's action on the carotid sinus baroreflex. The combination of fentanyl and diazepam has little influence on carotid sinus baroreflex control of the circulation in rabbits.