Habitual Exercise Training on Muscle Sympathetic Nerve Activity Responses to Hypercapnia in Older Adults

Abstract

Elevated activity of the sympathetic nervous system at rest and sympathetic hyperreactivity to stress have been shown to be predictors of cardiovascular diseases. The effect of habitual exercise training on muscle sympathetic reactivity to a stressor is unclear, with some studies demonstrating blunted muscle sympathetic nerve activity (MSNA) reactivity following training and others demonstrating no change in MSNA reactivity; however, many of these studies have not been performed in older adults. The purpose of this study was to compare MSNA responses to hypercapnia in older exercise trained and sedentary adults. Twenty-five older healthy adults were evaluated including 14 exercise trained (EX: age = 63 ± 4 years; BMI = 23 ± 3 kg/m2) and 11 sedentary controls (CON: age = 62 ± 6 years; BMI = 23 ± 2 kg/m2). Participants underwent a maximal exercise test to exhaustion on a cycle ergometer to measure VO2max. On a separate day, mean arterial pressure (MAP), heart rate (HR) and MSNA were measured continuously throughout the protocol. After baseline measurements were recorded, participants underwent stepped increases in hypercapnia, stabilizing at 6% CO2. Results are reported as mean ± SE. MAP was not different between groups at baseline (EX: 96 ± 2 mmHg vs. CON: 102 ± 5 mmHg; p>0.05) and during 6% CO2 (EX: 104 ± 4 mmHg vs. CON: 107 ± 5 mmHg; p>0.05). Both the change in MAP (EX: 8 ± 2 mmHg vs. CON: 5 ± 2 mmHg; p>0.05) and % change in MAP (EX: 8 ± 2 % vs. CON: 4 ± 2 %; p>0.05) were not different between groups. HR was not significantly different, but tended to be higher in the CON group at baseline (EX: 53 ± 2 bpm vs. CON: 57 ± 1 mmHg; p=0.09) and during 6% CO2 (EX: 57 ± 2 bpm vs. CON: 62 ± 2 bpm; p=0.09). Both the change in HR (EX: 4 ± 1 bpm vs. CON: 5 ± 1 mmHg; p>0.05) and % change in HR (EX: 8 ± 2 % vs. CON: 8 ± 2 %; p>0.05) were not different between groups. MSNA was not different between groups at baseline (EX: 32 ± 3 bursts/min vs. CON: 34 ± 3 bursts/min; p>0.05) and during 6% CO2 (EX: 35 ± 3 bursts/min vs. CON: 35 ± 3 bursts/min; p>0.05). Similarly, the change in MSNA (EX: 3 ± 1 bursts/min vs. CON: 1 ± 2 bursts/min; p>0.05) and the % change in MSNA (EX: 10 ± 5 % vs. CON: 5 ± 5 %; p>0.05) were not different between groups. The sympathetic response to hypercapnia was not different in older exercise-trained adults compared with sedentary adults. Taken together, these results suggest that hypercapnia-induced sympathoexcitation may not be altered with habitual exercise training in older healthy adults. Future studies could assess sympathoexcitation to other physiological stressors to further explore the effects of exercise on sympathetic reactivity in older adults.