Purpose: We tested the hypothesis that aging decreases the contribution of metaboreceptors to sympathetic responses during exercise in hypoxia.
Methods: 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.
Results: 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).
Conclusion: 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.