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 pressor reactivity during exercise, eight trained men [age, mean (SD), 24 (0.5) years; maximal oxygen uptake (VO2max), 4.7 l.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 isometric contraction at 30% maximum voluntary contraction until failure, and was monitored by a strain gauge interfaced from a leg extension apparatus to a computer. Heart rate (fc) and blood pressure (Pa) responses were recorded at rest, after 1 and 2 min of exercise, and at failure. Epinephrine (EPI) and norepinephrine (NE) levels were recorded before exercise, and after 2 min of exercise. The DYN exercise protocol involved stationary cycling for consecutive 6-min periods at 60% and 80% VO2max. fc, Pa, EPI, and NE were recorded before exercise and at each workload. No differences were observed in preexercise or exercise fc under any condition. Preexercise Pa did not differ under any condition. The Pa response to DYN was significantly higher at 80% VO2max during PM only. Pa was significantly higher in ST-PM at 1 min, 2 min, and failure. Elevations in both systolic and diastolic Pa contributed to this difference. Preexercise EPI-ST-AM was significantly elevated vs PM, but no other preexercise data were significantly different. Absolute exercise levels were significantly higher for EPI-ST-PM vs 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).(ABSTRACT TRUNCATED AT 250 WORDS)