Dissociation of maximal O2 uptake from O2 delivery in canine gastrocnemius in situ

Abstract

To test the hypothesis that maximal O2 uptake (V̇O2(max)) can be limited by O2 diffusion in the peripheral tissue, we kept O2 delivery [blood flow x arterial O2 content (Ca(O2))] to maximally contracting muscle equal between 1) low flow-high Ca(O2) and 2) high flow-low Ca(O2) conditions. The hypothesis predicts, because of differences in the capillary PO2 profile, that the former condition will result in both a higher V̇O2(max) and muscle effluent venous PO2 (PV(O2)). We studied the relations among V̇O2(max), PV(O2), and O2 delivery during maximal isometric contractions in isolated, in situ dog gastrocnemius muscle (n = 6) during these two conditions. O2 delivery was matched by varying arterial O2 partial pressure and adjusting flow to the muscle accordingly. A total of 18 matched O2 delivery pairs were obtained. As planned, O2 delivery was not significantly different between the two treatments. In contrast, V̇O2(max) was significantly higher [10.4 ± 0.5 (SE) ml · 100 g-1 · min-1; P = 0.01], as was PV(O2) (25 ± 1 Torr, P < 0.01) in the low flow-high Ca(O2) treatment compared with the high flow-low Ca(O2) treatment (9.1 ± 0.4 ml · 100 g-1 · min-1 and 20 ± 1 Torr, respectively). The rate of fatigue was greater in the high flow-low Ca(O2) condition, as was lactate output from the muscle and muscle lactate concentration. The results of this study show that V̇O2(max) is not uniquely dependent on O2 delivery and support the hypothesis that V̇O2(max) can be limited by peripheral tissue O2 diffusion.