EFFECTS OF EXERCISE TRAINING ON ALPHA-ADRENERGIC MEDIATED PRESSER RESPONSES AND BAROREFLEX FUNCTION IN OLDER SUBJECTS

Endurance exercise training increases maximal O-2 uptake (VO2 max) in older subjects, and training also improves cardiac function in older m en. Although the effects of training on beta-adrenergic responses have been investigated little information is available regarding the effec ts of cy-adrenergic responses and baroreflex function in older men and women. The purpose of the study was to determine whether endurance ex ercise training can affect ol-adrenergic responses and baroreflex func tion in the elderly. We studied 13 men and women, 63 +/- 4 yrs old (me an +/- SE). VO2 max was determined during treadmill exercise. Barorefl ex function was determined from the change in heart rate (HR) relative to the change in systolic blood pressure ((Delta HR/Delta SBP) during infusion of phenylephrine. VO2 max was increased by 23% (1.9 +/- 0.16 vs 2.34 +/- 0.20 l/min; p <.01) in response to training. Maximal hear t rate-did not change, but HR during submaximal exercise at the same a bsolute exercise intensity was 17% lower after training. Resting heart rate was slower in the trained state. During a-adrenergic stimulation induced by graded doses of phenylephrine infusion, heart rate was low er after training because of training-induced bradycardia at rest. How ever, the elevation in systolic blood pressure (Delta SBP) and mean bl ood pressure (Delta MBP) from basal levels in response to a given dose of phenylephrine were significantly larger (Delta SBP:18 +/- 3 vs26 /- 3 mmHg, p <.01; and Delta MBP 10 +/- 2 vs 15 +/- 3 mmHg, p <.01) af ter than before training. The doses of phenylephrine needed to raise s ystolic and diastolic blood pressure to comparable levels (SBP: 20 +/- 3 and 21 +/- 3 mmHg; DBP: 10 +/- 2 and 11 +/- 2 mmHg) were significan tly smaller (p <.025 after training (SBP 0.64 +/- 0.04 vs 0.44 +/- 0.0 7 mg/min; DBP: 0.67 +/- 0.04 vs 0.40 mg/min). The change in HR relativ e to the change in SBP (Delta HR/Delta SBP) was decreased from 0.67 +/ - .02 to .34 +/- .06 in response to training (p <.05). The results sug gest that endurance exercise training can enhance the a-adrenergic med iated vasopressor responses and reduce baroreflex function in older su bjects. The increased presser responses may protect against the possib le adverse effects of reduced baroreflex function in the elderly.