Increase in sympathetic activity with age. I. Role of impairment of arterial baroreflexes
Hajduczok, G.; Chapleau, M.W.; Johnson, S.L.; Abboud, F.M.
American Journal of Physiology 260(4 Pt 2): H1113-H1120
1991
ISSN/ISBN: 0002-9513 PMID: 1901459 Document Number: 382025
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 < 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.cntdot.min-1.cntdot.mmHg-1; P < 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. The results indicate that with aging 1) arterial baroreflex control of heart rate, renal sympathetic activity, and arterial pressure is significantly attenuated, 2) the impaired reflex represents a defect in control of both parasympathetic and sympathetic limbs, 3) there is a marked increase in resting RSNA that is much greater than expected from the impairment of arterial baroreflexes, and 4) the impaired baroreflex is not secondary to the elevated arterial pressure and cannot be accounted for by the increased baseline sympathetic activity.