INCREASE IN SYMPATHETIC ACTIVITY WITH AGE .2. ROLE OF IMPAIRMENT OF CARDIOPULMONARY BAROREFLEXES

We have previously shown that arterial baroreflex function is significantly impaired in a group of old beagles [G. Hajduczok, M. W. Chapleau, S. L. Johnson, and F. M. Abboud. Am. J. Physiol. 260 (Heart Circ. Physiol. 29): H1113-H1120, 1991]. In the present study, we determined whether the neural limb of the cardiopulmonary baroreflex control of renal sympathetic nerve activity (RSNA) and the interaction between the arterial and cardiopulmonary reflexes is also impaired with senescence. In the anesthetized state, the aortic nerves were sectioned and carotid sinuses were isolated bilaterally with carotid sinus pressures held at 50 mmHg in both young (1 yr) and old animals (11 yr). In response to graded volume expansion (VE) with 3% dextran (0 to 30 ml/kg iv), the gain of cardiopulmonary baroreflex control of RSNA was significantly lower in the old (5.1 +/- 3%/mmHg) compared with the young (13.4 +/- 3%/mmHg) animals. There were no significant differences in systemic vascular compliance or left ventricular mass index between the two groups. The gain of the carotid sinus baroreflex inhibition of RSNA was significantly attenuated by VE in the young group but was not affected in the old group. Bilateral vagotomy (VX) resulted in a 182 +/- 74% increase in RSNA in the young (P < 0.05), but VX did not significantly increase RSNA in the old (12 +/- 17%). After VX, the carotid sinus baroreflex gain markedly increased by nearly eightfold in the young group (P < 0.05) but was not altered in the old dogs. The results of the present study indicate that aging results in 1) an impaired cardiopulmonary baroreflex control of RSNA, 2) a decreased tonic vagal restraint on sympathetic nerve activity, 3) a loss of the inhibitory interaction between the cardiopulmonary and arterial baroreflexes, and 4) an augmented central sympathetic drive with age.