O2 gradients from sarcolemma to cell interior in red muscle at maximal VO2

The intracellular distribution of O2 in cross sections of dog gracilis muscles was determined by myoglobin (Mb) cryospectrophotometry. The volume sampled by the photometer was approximately 30 micron3 and contained 1-2 mitochondria. Measurements could be made to within 3 micron of capillaries without interference from hemoglobin. Mb saturation was uniform at all loci examined when respiration was blocked with cyanide. During twitch contraction at maximum O2 consumption, saturations within a cell cross section varied by up to 20%. The corresponding difference in partial pressure of O2 (PO2) was 1.5 Torr. Circumferential O2 gradients parallel to and 5 micron from the sarcolemma were greatest near capillaries. They did not exceed 0.1 Torr/micron and were dissipated within 25 micron of the sarcolemma. Gradients perpendicular to the sarcolemma were less than 0.02 Torr/micron. Saturation was not significantly correlated with cell diameter. Minimum PO2 was seldom located at the center of the cell cross section. Differences in saturation between contiguous cells often exceeded 10%. The distribution of O2 within cells appeared to reflect both an intercellular O2 flux and and an O2 flux from adjacent capillaries. Data agree qualitatively and quantitatively with mathematical models that take account of the particulate nature of blood and facilitated diffusion by Mb.