Intracellular PO2 in long axis of individual fibers in working dog gracilis muscle.

Dog gracilis muscles were frozen in situ during twitch contraction at 25-100% of aerobic capacity. O2 saturation of myoglobin (Mb) was determined from spectrophotometric measurements along individual fibers. Intracellular PO2 was calculated from the oxymyoglobin dissociation curve. At all work rates, long lengths of fibers were found in which saturation and PO2 were within the 4% error of measurement. During work at approximately 25% of aerobic capacity, Mb functioned at high saturation on the shallow slope of its dissociation curve. Consequently PO2 was poorly buffered. Although the range of saturation was small, PO2 varied up to 15 Torr along a fiber, and gradients up to 0.3 Torr/micron were observed. In contrast, at high O2 consumption (VO2), Mb functioned on the steep slope of its dissociation curve. Therefore gradients in intracellular PO2 along the axis of a myocyte were small (less than 0.05 Torr/micron) despite large gradients in Mb saturation (up to 0.5%/micron). Changes in intracellular PO2 over hundreds of microns did not reflect the large drop in intracapillary PO2 between arterioles and venules. Because intracellular PO2 is low and relatively uniform in the long axis of a fiber, the driving force for release of O2 from blood is dominated by intravascular PO2 in working red muscle.