ANP enhances bradycardic reflexes in normotensive but not spontaneously hypertensive rats

Baroreflex control of heart rate in spontaneously hypertensive rats (SHR) is defective, largely because of a poor vagal contribution to the reflex. We have demonstrated previously that atrial natriuretic peptide (ANP) enhances reflex bradycardia in normotensive rats through an action on nonarterial vagal afferent pathways. In the present study, we investigated whether ANP could reverse the baroreflex abnormality in SHR. Heart rate reflexes were activated by three different methods in conscious, instrumented SHR and Wistar-Kyoto rats (WKY) in the presence of intravenous infusions of vehicle (saline) or rat ANP (150 ng/kg per minute). Heart rate responses were measured by (1) the steady-state changes in blood pressure after alternating slow infusions (over approximately 15 to 30 seconds) of a pressor (methoxamine) and depressor (nitroprusside) drug (stimulating predominantly arterial baroreceptors), (2) the ramp method of rapid infusion of methoxamine (over < 10 seconds; stimulating arterial and cardiopulmonary baroreceptors), and (3) the von Bezold-Jarisch method of activating chemically sensitive cardiac receptors through serotonin injections. ANP enhanced the heart rate range of the arterial baroreflex (steady-state method) by 13 +/- 3% in WKY but had no significant effect on the sensitivity or any other parameter of the steady-state baroreflex. When a very rapid rise in blood pressure was elicited by the ramp method in WKY, ANP significantly enhanced baroreflex bradycardia (sensitivity increased by 29 +/- 9%, P < .05). ANP also enhanced the bradycardia of the von Bezold-Jarisch reflex (by 33 +/- 16%, P < .05) in WKY. By contrast, ANP did not influence baroreceptor or chemoreceptor heart rate reflex responses in SHR. We conclude that in normotensive rats, ANP facilitates cardiopulmonary bradycardic reflexes. The lack of effect of ANP in SHR may be related to an underlying structural or genetic alteration in their cardiac sensors, perhaps associated with cardiac hypertrophy, that prevents the ANP-induced activation of cardiac sensory afferents, resulting in cardioinhibition.