ANP potentiates nonarterial baroreflex bradycardia: evidence from sinoaortic denervation in rats

Previous findings indicate that atrial natriuretic peptide (ANP) enhances the reflex bradycardia arising from stimulation of cardiac mechanoreceptors and chemoreceptors, but not that from arterial baroreceptors. The present study tests this proposal by examining the effect of ANP on these reflexes in six chronically sinoaortic-denervated (SAD), and eight sham-operated (sham), conscious rats. Arterial baroreceptor-heart rate (HR) reflex function was examined by constructing full-range steady-state blood pressure (BP)-HR curves using alternating doses of pressor (methoxamine, 2–100 μg/kg) and depressor (nitroprusside, 1–50 μg/kg) agents. Nonarterial baroreceptor reflex function was assessed by the ‘ramp’ bradycardic response to the rapid BP rise after i.v. methoxamine (100 μg/kg bolus dose). The cardiopulmonary chemoreflex was evoked by i.v. injections of serotonin (1–20 μg/kg). These three tests were performed on each rat during infusions, in random order, of rat ANP (150 ng/kg/min i.v.) and saline vehicle. The ability of ANP to significantly enhance ramp reflex bradycardia was not diminished in SAD compared with sham rats (+54±12% vs. +42±15%, respectively). ANP also significantly enhanced cardiopulmonary chemoreflex bradycardia in both groups (+60±15% in SAD, +40±8% in sham). Neither the normal steady-state BP-HR response in sham rats nor the small residual response in SAD rats was enhanced by ANP (−1±7% in sham, −11±8% in SAD). We conclude that ANP enhances reflex bradycardias of nonarterial, probably cardiac mechanoreceptor, origin.