Increase in sympathetic activity with age. I. Role of impairment of arterial baroreflexes.

The purpose of this study was to evaluate changes in arterial baroreflex function with aging. Mean arterial pressure (MAP) obtained in the awake state was 110 +/- 7 mmHg in the young animals (1 yr; n = 5) and 128 +/- 6 mmHg in the old beagles (11 yr; n = 11) (P less than 0.05). In response to bolus administration of varying doses of phenylephrine and nitroglycerin in the conscious state, the slope relating heart rate (HR) to MAP was attenuated significantly in the old animals compared with the young (-0.87 +/- 0.30 vs. -2.35 +/- 0.44 beats.min-1.mmHg-1; P less than 0.05). After atropine, the baroreflex control of HR was abolished in both groups. After anesthesia and sectioning of the aortic depressor nerves, and with isolated carotid sinus pressures (CSP) held at 50 mmHg, absolute renal sympathetic nerve activity (RSNA) was significantly greater in the old (368 +/- 40 Hz) vs. the young animals (41 +/- 9 Hz). In the old, the gains of baroreflex inhibition of MAP (0.78 +/- 0.09) and normalized RSNA (0.38 +/- 0.14%/mmHg) during increases in CSP were decreased significantly compared with the young (MAP, 1.16 +/- 0.17 mmHg, and RSNA, 0.72 +/- 0.06%/mmHg). In a subgroup of old normotensive animals (n = 5), the baroreflex gain of RSNA was still attenuated (0.43 +/- 0.11%/mmHg) compared with the young. The reflex reduction in absolute RSNA as a function of baseline RSNA was also impaired in old vs. young beagles.(ABSTRACT TRUNCATED AT 250 WORDS)