ENDOGENOUS BRADYKININ IN THE THORACIC SPINAL-CORD CONTRIBUTES TO THE EXERCISE PRESSOR REFLEX

This investigation tested the hypothesis that bradykinin causes excita tory effects in the thoracic spinal cord that augment the exercise pre sser reflex. Thus we performed 30 s of electrically stimulated static contraction of the hindlimb in the anesthetized cat (alpha-chloralose) to provoke reflex-induced increases in mean arterial pressure, maxima l rate of rise of left ventricular pressure (dP/dt), and heart rate (i .e., the exercise presser reflex. These three responses were compared before and 15 min after intrathecal injection of 2 mu g (n = 3), 10 mu g (n = 6), or 50 mu g (n = 3) of the selective bradykinin B-2-recepto r antagonist HOE-140 into the thoracic spinal cord or 10 mu g of this antagonist into the lumbar (n = 3) spinal cord. In three of the six ca ts in which 10 mu g of HOE-140 were injected into the thoracic spinal cord, an additional contraction was performed 60-90 min after treatmen t. The 2-mu g dose of HOE-140 had no effect on the exercise pressor re flex. Injection of 10 mu g of this antagonist into the thoracic spinal cord reduced the contraction-evoked presser, maximal dP/dt, and heart rate responses by 49 +/- 7, 58 +/- 4, and 64 +/- 13%, respectively (P < 0.05). Fifty micrograms of HOE-140 failed to attenuate these respon ses further. In the three cats in which an additional contraction was performed 60-90 min after treatment with 10 mu g of the antagonist, bl ood pressure and dP/dt responses had returned, in part, toward initial values. Neither intravenous (n = 3) nor intrathecal injection of 10 m u g of HOE-140 into the lumbar spinal cord had agr effect oil tile con traction-induced cardiovascular responses. Thoracic injection of 50-20 0 ng of bradykinin provoked a presser response of 26 +/- 5 mmHg that w as abolished by a similar injection of 10 mu g of ROE-140. These data suggest that endogenous bradykinin contributes to the exercise presser reflex by an excitatory action in the thoracic spinal cord.