Effects of exercise training on α-adrenergic mediated pressor responses and baroreflex function in older subjects

Endurance exercise training increases maximal O₂ uptake (V̇O₂max) in older subjects, and training also improves cardiac function in older men. Although the effects of training on β-adrenergic responses have been investigated, little information is available regarding the effects of α- adrenergic responses and baroreflex function in older men and women. The purpose of the study was to determine whether endurance exercise training can affect α-adrenergic responses and baroreflex function in the elderly. We studied 13 men and women, 63 ± 4 yrs old (mean ± SE). V̇O₂max was determined during treadmill exercise. Baroreflex function was determined from the change in heart rate (HR) relative to the change in systolic blood pressure (ΔHR/ΔSBP) during infusion of phenylephrine. V̇O₂max was increased by 23% (1.9 ± 0.16 vs 2.34 ± 0.20 l/min; p < .01) in response to training. Maximal heart rate did not change, but HR during submaximal exercise at the same absolute exercise intensity was 17% lower after training. Resting heart rate was slower in the trained state. During α- adrenergic stimulation induced by graded doses of phenylephrine infusion, heart rate was lower after training because of training-induced bradycardia at rest. However, the elevation in systolic blood pressure (ΔSBP) and mean blood pressure (ΔMBP) from basal levels in response to a given dose of phenylephrine were significantly larger (ΔSBP: 18 ± 3 vs 26 ± 3 mmHg, p < .01; and ΔMBP 10 ± 2 vs 15 ± 3 mmHg, p < .01) after than before training. The doses of phenylephrine needed to raise systolic and diastolic blood pressure to comparable levels (SBP: 20 ± 3 and 21 ± 3 mmHg; DBP: 10 ± 2 and 11 ± 2 mmHg) were significantly smaller (p < .025 after training (SBP 0.64 ± 0.04 vs 0.44 ± 0.07 mg/min; DBP: 0.67 ± 0.04 vs 0.40 mg/min). The change in HR relative to the change in SBP (ΔHR/ΔSBP) was decreased from .67 ± .02 to .34 ± .06 in response to training (p < .05). The results suggest that endurance exercise training can enhance the α-adrenergic mediated vasopressor responses and reduce baroreflex function in older subjects. The increased pressor responses may protect against the possible adverse effects of reduced baroreflex function in the elderly.