TIME OF DAY EFFECTS ON SYMPATHOADRENAL AND PRESSOR REACTIVITY TO EXERCISE IN HEALTHY-MEN

To investigate the influence of time of day on sympathoadrenal and pre ssor reactivity during exercise, eight trained men [age, mean (SD), 24 (0.5) years; maximal oxygen uptake (VO2max), 4.71 . min-1] performed bouts of static (ST) and dynamic (DYN) exercise at 0600-0800 hours (AM ) and at 1600-1800 hours (PM). The ST protocol utilized a two-leg isom etric contraction at 30% maximum voluntary contraction until failure, and was monitored by a strain gauge interfaced from a leg extension ap paratus to a computer. Heart rate (f(c)) and blood pressure (P(a)) res ponses were recorded at rest, after 1 and 2 min of exercise, and at fa ilure. Epinephrine (EPI) and norepinephrine (NE) levels were recorded before exercise, and after 2 min of exercise. The DYN exercise protoco l involved stationary.cycling for consecutive 6-min periods at 60% and 80% VO2max. f(c), P(a). EPI, and NE were recorded before exercise and at each workload. No differences were observed in preexercise or exer cise f(c) under any condition. Preexercise P(a) did not differ under a ny condition. The Pa response to DYN was significantly higher at 80% V O2max during PM only. Pa was significantly higher in ST-PM at 1 min, 2 min, and failure. Elevations in both systolic and diastolic P(a) cont ributed to this difference. Preexercise EPI-ST-AM was significantly el evated vs PM, but no other preexercise data were significantly differe nt. Absolute exercise levels were significantly higher for EPI-ST-PM v s AM only, but the percentage change from baseline was significantly ( P<0.01) higher in ST-PM for EPI (+ 231% PM vs + 32% AM) and NE (+ 352% PM vs + 216% AM). The EPI and NE responses to DYN exercise tended to be higher in AM, but were not significantly different. These data supp ort a time of day pattern in sympathoadrenal and pressor reactivity to exercise that is dependent on the type of activity involved but indep endent of baseline patterns.