EFFECT OF HEMOGLOBIN CONCENTRATION ON MAXIMAL O2 UPTAKE IN CANINE GASTROCNEMIUS-MUSCLE INSITU

O2 delivery to maximally working muscle was decreased by altering hemoglobin (Hb) concentration and arterial PO2 (Pa(O2)) to investigate whether the reductions in maximal O2 uptake (VO2max) that occur with lowered [Hb] are in part related to changes in the effective muscle O2 diffusing capacity (Dm(O2)). Two sets of experiments were conducted. In the initial set (n = 8), three levels of Hb [5.8 +/- 0.3, 9.4 +/- 0.1, and 14.4 +/- 0.6 (SE) g/100 ml] in the blood were used in random order to pump perfuse, at equal muscle blood flows and Pa(O2), maximally working isolated dog gastrocnemius muscle. VO2max declined with decreasing [Hb], but the relationship between VO2max and both the effluent venous PO2 (Pv(O2)) and the calculated mean capillary PO2 (PcBAR(O2)) was not linear through the origin and, therefore, not compatible with a single value of Dm(O2) (as calculated by Bohr integration using a model based on Fick's law of diffusion). To clarify these results, a second set of experiments (n = 6) was conducted in which two levels of Hb (14.0 +/- 0.6 and 6.9 +/- 0.6 g/100 ml) were each combined with two levels of oxygenation (Pa(o2) 79 +/- 8 and 29 +/- 2 Torr) and applied in random sequence to again pump perfuse maximally working dog gastrocnemius muscle at constant blood flow. In these experiments, the relationship between VO2max and both Pv(O2) and calculated PcBAR(O2) for each [Hb] was consistent with a constant estimate of Dm(O2) as Pa(O2) was reduced, but the calculated Dm(O2) for the lower [Hb] was 33% less than that at the higher [Hb] (P < 0.05). Whether the apparent reduction in Dm(O2) with lower [Hb] is due to increased erythrocyte spacing, reduced chemical off loading of O2 from Hb, or some other effect remains to be determined.