On the mechanisms that limit oxygen uptake during exercise in acute and chronic hypoxia: role of muscle mass

Peak aerobic power in humans ((V) over dot(O2), peak) is markedly affected by inspired O-2 tension (F-IO2). The question to be answered in this study is what factor plays a major role in the limitation of muscle peak. (V) over dot(O2) in hypoxia: arterial O-2 partial pressure (P-a,(O2)) or O-2 content (C-a,C-O2)? Thus, cardiac output (dye dilution with Cardio-green), leg blood flow (thermodilution), intra-arterial blood pressure and femoral arterial-to-venous differences in blood gases were determined in nine lowlanders studied during incremental exercise using a large (two-legged cycle ergometer exercise: Bike) and a small (one-legged knee extension exercise: Knee) muscle mass in normoxia, acute hypoxia (AH) (F-IO2 = 0.105) and after 9 weeks of residence at 5260 m (CH). Reducing the size of the active muscle mass blunted by 62% the effect of hypoxia on. (V) over dot(O2,peak) in AH and abolished completely the effect of hypoxia on. (V) over dot(O2,peak) after altitude acclimatization. Acclimatization improved Bike peak exercise Pa, O-2 from 34 +/- 1 in AH to 45 +/- 1 mmHg in CH(P < 0.05) and Knee Pa, O-2 from 38 +/- 1 to 55 +/- 2 mmHg(P < 0.05). Peak cardiac output and leg blood flow were reduced in hypoxia only during Bike. Acute hypoxia resulted in reduction of systemic O-2 delivery (46 and 21%) and leg O-2 delivery (47 and 26%) during Bike and Knee, respectively, almost matching the corresponding reduction in. (V) over dot(O2,peak). Altitude acclimatization restored fully peak systemic and leg O-2 delivery in CH (2.69 +/- 0.27 and 1.28 +/- 0.11 l min(-1), respectively) to sea level values (2.65 +/- 0.15 and 1.16 +/- 0.11 l min(-1), respectively) during Knee, but not during Bike. During Knee in CH, leg oxygen delivery was similar to normoxia and, therefore, also. (V) over dot(O2,peak) in spite of a P-a,P-O2 of 55 mmHg. Reducing the size of the active muscle mass improves pulmonary gas exchange during hypoxic exercise, attenuates the Bohr effect on oxygen uploading at the lungs and preserves sea level convective O-2 transport to the active muscles. Thus, the altitude-acclimatized human has potentially a similar exercising capacity as at sea level when the exercise model allows for an adequate oxygen delivery (blood flow x C-a,C-O2), with only a minor role of P-a,P-O2 per se, when P-a,P-O2 is more than 55 mmHg.