Attenuation of baroreceptive mechanisms by cardiovascular sympathetic afferent fibers.

By sectioning spinal dorsal roots from C8 to T6, we analyzed the contribution of sympathetic cardiovascular afferent fibers to the reflex bradycardia induced by arterial pressure rises in 24 anesthetized and in 21 decerebrate cats. In anesthetized cats, the reflex bradycardia was obtained in 16 animals with occlusions of the thoracic aorta and in 8 animals with phenylephrine injections (25-75 micrograms/kg). In both experimental conditions, the dorsal root section enhanced the bradycardia response, which thus increased from 15 +/- 3 to 20 +/- 3% during aortic constrictions and from 11 +/- 3 to 19 +/- 6% during phenylephrine injections (P less than 0.05). The enhancement, after rhizotomy of the reflex bradycardia during aortic occlusion, was more pronounced in eleven decerebrate cats as it increased from 21 +/- 4 to 34 +/- 4%, P less than 0.05. In five vagotomized and decerebrate cats, the reflex bradycardia was also increased after rhizotomy despite the overall reduction of the reflex response. In five decerebrate cats with beta-adrenergic receptor blockade (propranolol 0.2-0.4 mg/kg iv), aortic occlusion resulted in a small reduction in heart rate which was not significantly affected by dorsal root section. Our data indicate that excitatory reflexes mediated by dorsal roots are likely to modulate the inhibitory supraspinal reflexes that determine the heart rate reduction during acute rises in arterial blood pressure.