Effects of fentanyl on a bradykinin-induced excitatory cardiac reflex in the anaesthetized rat.

A reflex increase in arterial blood pressure and heart rate was elicited by application of epicardial bradykinin (4 X 10(-8), 2 X 10(-7), 10(-6) g/ml) in pentobarbital-anaesthetized, thoracotomized rats. The influence of the opioid fentanyl and the tranquilizer diazepam on this increase was then investigated. When fentanyl (2.2 micrograms/(kg X min] was infused i.v. during periods of bradykinin application, a highly significant suppression of the excitatory reflex resulted. Epicardial bradykinin (10(-6) g/ml) induced an increase in mean arterial blood pressure of 25.3 +/- 3.5 mm Hg during the control period and of 6.8 +/- 1.4 mm Hg during fentanyl infusion. The corresponding changes in heart rate were +20.8 +/- 4.7 beats/min during the control period and +1.9 +/- 1.3 beats/min during fentanyl infusion. When cardioacceleration was induced by the carotid occlusion reflex, fentanyl antagonized this effect too. The effect of fentanyl was entirely neutralized by injecting naloxone (30 micrograms/kg i.v.) prior to application of the former. Diazepam had no influence on the bradykinin-induced reflex. The results support the view that morphinoceptors may modulate the excitatory cardiac reflex induced by epicardial application of the algesic bradykinin in the rat.