Epicardial bradykinin B2 receptors elicit a sympathoexcitatory reflex in rats

Bradykinin may be generated in the heart during ischemia and is involved in nociception. We tested the hypothesis that bradykinin elicits a sympathoexcitatory reflex in rats by stimulating cardiac afferent nerve fibers. Rats were implanted with femoral catheters for measurement of blood pressure and heart rate, a bipolar electrode for measurement of renal sympathetic nerve activity, and a pericardial catheter for intrapericardial injection of substances. Rats were slightly anesthetized with hexobarbital so pain reactions were prevented. Graded doses of bradykinin (2.5, 12, 25 micrograms) were injected intravenously or intrapericardially into control rats, intrapericardially after vagotomy, intrapericardially after intrapericardial pretreatment with the bradykinin B2 receptor antagonist Hoe 140, and intrapericardially after cardiac autonomic blockade (intrapericardial pretreatment with 10% procaine). For comparison, the serotonin 5-HT3 agonist phenylbiguanide, a substance known to elicit sympathoinhibitory reflexes by cardiac vagal afferents, and adenosine, putatively inducing sympathoexcitatory responses via the heart, were applied intrapericardially. Bradykinin increased blood pressure when administered intrapericardially but decreased blood pressure when injected intravenously; both intrapericardial and intravenous bradykinin increased renal sympathetic nerve activity. Intrapericardial adenosine had no effect on circulatory control. Intrapericardial pretreatment with the B2 receptor antagonist Hoe 140 completely inhibited the increases of blood pressure and renal sympathetic nerve activity in response to intrapericardial bradykinin but did not affect the responses to intrapericardial phenylbiguanide. Bilateral cervical vagotomy abolished the decreases of blood pressure, heart rate, and renal sympathetic nerve activity after intrapericardial phenylbiguanide but did not influence the responses to intrapericardial bradykinin. Cardiac autonomic blockade with intrapericardial procaine abolished all responses to bradykinin and phenylbiguanide. We conclude that cardiac bradykinin elicits a sympathoexcitatory reflex by epicardial B2 receptors in rats. The afferent portion of the reflex is most likely contained within sympathetic cardiac afferent fibers. Bradykinin may contribute to increased sympathetic nerve activity in pathophysiological situations of coronary artery disease and cardiac ischemia.