BETA-ADRENERGIC-BLOCKADE DOES NOT PREVENT THE LACTATE RESPONSE TO EXERCISE AFTER ACCLIMATIZATION TO HIGH-ALTITUDE

We examined the extent to which epinephrine influences blood lactate a djustments to exercise during both acute (AC) and chronic (CH) high-al titude exposure. Eleven male sea level residents were divided into a c ontrol group (n = 5) receiving a placebo or a drug group (n = 6) recei ving 240 mg/day of propranolol. All subjects were studied at rest and during 45 min of submaximal exercise (similar to 50% of sea level maxi mal O-2 uptake) at sea level (SL) and within 4 h of exposure to and af ter 3 wk residence at 4,300 m (summit of Pikes Peak). Blood samples we re collected from the femoral artery for epinephrine and lactate conce ntration. Exercising blood lactate concentration was significantly dif ferent across all altitude conditions such that AC > CH > SL (P < 0.05 ). For a given arterial O-2 saturation, mean exercising blood lactates were lower for the beta-blocked group compared with controls; however , both groups demonstrated similar patterns across all conditions. Epi nephrine levels during exercise followed a similar pattern to that of lactate, averaging 0.67, 0.43, and 0.29 ng/ml for AC, CH, and SL, resp ectively. The correlation between lactate and epinephrine was 0.93 and 0.84 for control and beta-blocked subjects, respectively. Whereas dur ing exercise epinephrine was consistently higher for the beta-blocked group than controls, this difference was only significant during CH ex posure. The epinephrine response was related to the extent of hypoxia in both groups. It was concluded that whereas the sympathoadrenal resp onse has an integral role in the adjustments made to exercise at altit ude, this response does not account entirely for the lactate changes o bserved during exercise at high altitude or after acclimatization.