DETERMINANTS OF MAXIMAL OXYGEN-UPTAKE IN RATS ACCLIMATED TO SIMULATED ALTITUDE

The effect of acclimation to hypoxia on maximal O2 uptake (Vo2 max), maximal cardiac output (Qmax), and arteriovenous O2 concentration difference (a-vCo2) was studied in male Sprague-Dawley rats acclimated for 3 wk to a barometric pressure of approximately 380 Torr (A rats). Nonacclimated control animals were pair-fed littermates maintained at an ambient barometric pressure of approximately 740 Torr (NA rats). Both A and NA rats exercised maximally on a treadmill with inspired Po2 maintained at either approximately 72 or 145 Torr. Arterial blood O2 concentration was significantly higher in A than in NA rats (16.0 +/- 0.6 vs. 12.4 +/- 0.3 ml/dl in hypoxia and 28.4 +/- 1.5 vs. 20.1 +/- 0.9 ml/dl in normoxia, respectively; both P < 0.05). During hypoxic exercise Vo2 max was slightly but significantly higher in A than in NA subjects (55.3 +/- 1.3 vs. 48.8 +/- 1.1 MI STPD . min-1 . kg-1; P < Q.05). In hypoxia a-vCo2 was 16.6 +/- 0.6 and 12.4 +/- 0.4 ml/dl and Qmax was 401 +/- 17 and 489 +/- 9 ml . min-1 . kg-1 in A and NA subjects, respectively (both P < 0.05). A rats showed both lower maximal heart rate and lower maximal stroke volume during hypoxic exercise. In normoxia there, was no significant difference in Vo2 max between A and NA rats (71.8 +/- 2.7 vs. 73.9 +/- 3.1 ml . min-1 . kg-1). As with hypoxia, in normoxia a-VCo2 was significantly higher and Qmax was significantly lower in A than in NA animals. These data show that one reason for the modest increase in Vo2 max in prolonged hypoxia is that the increase in arterial blood O2 concentration is offset by a decrease in Qmax such that the product, O2 transport, remains relatively unchanged. The proportional relationships between Vo2 max and O2 transport and between Vo2 max and venous Po2 do not appear to be different in A and in NA animals. These findings suggest that factors other than convective O2 transport to muscle do not contribute to the small increase in Vo2 max in prolonged hypoxia.