Aging and baroreflex control of RSNA and heart rate in rats

Aging is associated with altered autonomic control of cardiovascular functi on, but baroreflex function in animal models of aging remains controversial . In this study, pressor and depressor agent-induced reflex bradycardia and tachycardia were attenuated in conscious old (24 mo) rats [57 and 59% of r esponses in young (10 wk) Wistar rats, respectively]. The intrinsic heart r ate (HR, 339 +/- 5 vs. 410 +/- 10 beats/min) was reduced in aged animals, b ut no intergroup differences in resting mean arterial blood pressure (MAP, 112 +/- 3 vs. 113 +/- 5 mmHg) or HR (344 +/- 9 vs. 347 +/- 9 beats/min) exi sted between old and young rats, respectively. The aged group also exhibite d a depressed (49%) parasympathetic contribution to the resting HR value (v agal effect) but preserved sympathetic function after intravenous methylatr opine and propranolol. An implantable electrode revealed tonic renal sympat hetic nerve activity (RSNA) was similar between groups. However, old rats s howed impaired baroreflex control of HR and RSNA after intravenous nitropru sside (-0.63 +/- 0.18 vs. -1.84 +/- 0.4 bars.cycle(-1).mmHg(-1).s(-1)). The refore, aging in rats is associated with 1) preserved baseline MAP, HR, and RSNA, 2) impaired baroreflex control of HR and RSNA, and 3) altered autono mic control of resting HR.