GAS-EXCHANGE, BLOOD LACTATE, AND PLASMA-CATECHOLAMINES DURING INCREMENTAL EXERCISE IN HYPOXIA AND NORMOXIA

The interrelationships among blood lactate (La-) and plasma norepinephrine (NE) and epinephrine (Epi) were studied simultaneously with measures of ventilation (VE) and gas exchange during incremental exercise to exhaustion in nine healthy young men. We wanted to observe whether the tight coupling that exists during normoxic exercise between the concentrations of La- ([La-]) and of both NE and Epi would also be found in hypoxia (inspired O-2 fraction = 0.14). In addition, we used recently advocated methods of V slope [CO2 output vs. O-2 uptake (VO2)] to select the ventilatory threshold (VT) and log-log transformation of [La-] and VO2 to select the lactate threshold (LT). Peak VO2 was reduced from 4,164 +/- 184 ml/min in normoxia to 3,635 +/- 144 ml/min in hypoxia (P < 0.05). The increase in [La-] was linearly related to the increases in both NE and Epi concentrations in the normoxic and hypoxic tests (r = 0.92-0.96). Estimates of VO2 at VT were significantly greater than those at LT in both normoxia and hypoxia, but these estimates were poorly correlated (r = -0.11-0.46). VT and LT were reduced by hypoxia. Visual interpretation of the VT by examination of VE vs. VO2 and VE/VO2 vs. VO2 did not differ from the LT, but they were less than the VTs by the V-slope method (P < 0.05); yet, all were poorly correlated. The tight coupling between the increase in [La-] and the increase in plasma catecholamines might indicate a common mechanism for the increase or a causative link. VT and LT provided estimates of the general trend in the data, but the poor correlation between them questions the utility of attempting to predict one from the other.