We tested the hypothesis that aging decreases the contribution of metaboreceptors to sympathetic responses during exercise in hypoxia.
We recorded sympathetic nerve traffic to muscle circulation (MSNA), heart rate (HR), blood pressure (BP), minute ventilation (VE), and blood lactate (BL) in 12 older (55 ± 10 yr) and 12 younger (22 ± 2 yr) normal subjects during three randomized interventions: isocapnic hypoxia (chemoreflex activation), isometric handgrip exercise (HG) in normoxia (metaboreflex activation), and HG during isocapnic hypoxia (concomitant metaboreflex and chemoreflex activation). All interventions were followed by a forearm circulatory arrest period to allow metaboreflex activation in the absence of exercise and chemoreflex activation.
Older subjects had higher resting MSNA (38 ± 12 vs 23 ± 9 bursts per minute; P < 0.01) and BP (P < 0.001). Heart rate, minute ventilation, and blood lactate did not differ (all P > 0.5). MSNA responses to HG in normoxia (P < 0.05) and in hypoxia (P < 0.05) were smaller in the older subjects, but were similar during hypoxia alone. The increase in HR was smaller in the older subjects for all interventions (all P < 0.05). In contrast, the increase in systolic and diastolic BP, VE, and BL were similar in both groups (P > 0.05). During the local circulatory arrest, MSNA and BP remained elevated in both groups after HG in normoxia (P < 0.01) and in hypoxia (P < 0.01), but MSNA changes were smaller in the older subjects (P < 0.05).
Aging reduces sympathetic reactivity to isometric handgrip, but does not prevent the metaboreceptors to remain the main determinant of sympathetic activation during exercise in hypoxia.
1Department of Physiology, Erasme Hospital, Brussels, BELGIUM; and 2Department of Cardiology, Erasme Hospital, Brussels, BELGIUM
Address for correspondence: R. Naeije, Department of Physiology, Erasme University Hospital, 808 Lennik road, B-1070 Brussels, Belgium; E-mail: firstname.lastname@example.org.
Submitted for publication May 2005.
Accepted for publication September 2005.