Physiological and pharmacological properties of the three subsystems constituting the aortic nerve-renal sympathetic reflex in rabbits.

Electrical stimulation of the aortic nerve of anesthetized rabbits reflexly evoked both excitation and inhibition of renal nerve activity. The excitatory component of the reflex, observed in about 75% of the animals, was elicited by activation of aortic C-fibers. It was selectively suppressed by chronic treatment of the animal with capsaicin. Intracisternal injection of either [D-ala2]-met-enkephalinamide or beta-endorphin markedly attenuated this excitatory component, although neither affected the excitatory component mediated by chemoreceptor fibers in response to stimulation of the carotid sinus nerve. It seems most likely that nociceptive C-fibers of the rabbit's aortic nerve were responsible for the excitatory component. On the other hand, the inhibitory component was reflexly elicited by stimulation of the aortic A- or C-fiber group activated separately or in combination. In agreement with previous reports, the sympatho-inhibitory action of C-fibers was more powerful and longer-lasting than that of A-fibers. We found that the inhibitory component induced by C-fibers was markedly attenuated by the two opioid peptides mentioned above, but was resistant to pentobarbital. On the contrary, the component mediated by A-fibers was suppressed by pentobarbital but was relatively resistant to the opioid peptides. Thus, the rabbit's aortic nerve-renal sympathetic reflex consists of the following 3 subsystems characterized by different physiological and pharmacological properties: sympatho-inhibitory systems activated by barosensory A- or C-fibers and a sympatho-excitatory system attributable to C-fibers probably of nociceptive modality.