SYMPATHETIC NEURAL ADJUSTMENTS TO STRESS IN PHYSICALLY TRAINED AND UNTRAINED HUMANS

The purpose of this study was to determine if the state of physical training influences sympathetic neural activation during acute stress in humans. We recorded muscle sympathetic nerve activity (microneurography of the peroneal nerve), arterial blood pressure, and heart rate in 12 highly trained, endurance athletes (25+/-1 years, mean+/-SEM) and 12 untrained subjects (27+/-1 years) before (supine rest control) and during: 1) lower body negative pressure at -5, -10, -15, and -20 mm Hg (orthostatic stress); 2) isometric handgrip at 30% of maximum (exercise stress); and 3) hand immersion in ice water, that is, the cold pressor test (thermal stress). Body weight was not different in the two groups, but the athletes had a lower body fat content (8.9+/-1.3% versus 16.1+/-2.0%, p < 0.05). During supine rest, muscle sympathetic nerve burst frequency (24+/-3 versus 24+/- bursts/min, athletes versus untrained subjects) and burst incidence (36+/-3 versus 44+/-4 bursts/100 heart beats) and aterial blood pressure were not different in the two groups, but heart rate was lower in the athletes (54+/-2 versus 67+/-3 beats/min, p < 0.05). During graded lower body negative pressure, mean arterial pressure and heart rate did not change from control levels in either group, but muscle sympathetic nerve activity increased in a stimulus-dependent manner in both groups; the changes in total minute activity (% control) in the athletes (113+/-8, 136+/-11, 174+/-12, and 209+/-20) and the untrained subjects (132+/-6, 163+/-8, 210+/-19, and 262+/-30) were not different. The magnitudes of increases in total minute muscle sympathetic nerve activity in response to isometric handgrip (161+/-15 versus 204+/-34% control) and the cold pressor test (300+/-29 versus 255+/-39% control) was not different in the two groups nor were the associated changes in arterial pressure and heart rate. The results indicate that the sympathetic neural and cardiovascular adjustments to these common laboratory stimuli are not significantly influenced by the individual's level of aerobic conditioning. In addition, the data confirm the previous finding that the state of physical training does not affect muscle sympathetic nerve activity at rest. Thus, this factor does not appear to be a primary contributor to the marked intersubject variability observed in sympathetic nerve discharge at rest and in response to acute physical stress in humans.