Influence of age on the sympathetic neural adjustments to alterations in systemic oxygen levels in humans

We tested the hypothesis that aging influences the regulation of sympathetic nervous system activity (SNA) and arterial blood pressure during alterations in systemic O-2 levels in humans. To accomplish this, are performed direct (intraneural) measurements of SNA to skeletal muscle (MSNA) in 10 young and 7 older healthy normotensive men during room air breathing (normoxic control), moderate isocapnic hypoxemia [15 min of 10% fractional inspired O-2 (FIO2)], and hyperoxemia (10 min of 50% FIO2). After hypoxemia, arterial O-2 saturation (SaO(2)) declined similarly in the young and older men. MSNA (burst frequency and total minute activity) increased significantly (P < 0.05) in both groups. The magnitudes of the absolute increases in MSNA and the Delta MSNA/Delta SaO(2) were not significantly different in the young and older men; however, because of higher normoxic baseline levels, the percentage increases in burst frequency were smaller (P = 0.02) and those for total minute activity tended to be smaller (P = 0.11) in the older men. Arterial blood pressure increased modestly (P < 0.05) and similarly in both groups, although the older men demonstrated a smaller increase in heart rate. After hypoxemia, SaO(2) increased and MSNA decreased (both P < 0.05) similarly in the young and older men. Arterial blood pressure did not change significantly from normoxic control levels in either group; however, a small (P < 0.05) reduction in heart rate was observed in both groups. In conclusion, aging does not obviously influence the regulation of absolute levels of MSNA or arterial blood pressure during alterations in systemic O-2 levels in healthy men, although older men demonstrate a smaller percentage increase in MSNA from their elevated baseline levels, as well as an attenuated tachycardia in response to acute hypoxemia. As such, the present results are consistent with our previous findings on aging and sympatho-circulatory control during other types of acute stress in humans.