COBURN, RF, AND LB MAYERS. Myoglobin 02 tension determined from measurements of carboxymyoglobin in skeletal muscle. Am. J. Physiol. 220 (l): 66-74. 1971.-We have developed a method of estimating muscle intracellular oxygen tension in vivo, using myoglobin as an oxygen tension indicator. The method is based on the competition of oxygen and carbon monoxide for binding sites on myoglobin and allows calculation of the mean 02 tension in equilibrium with myoglobin from measurements of mean muscle PCO and carboxymyoglobin percent saturation. In our experiments the partition of CO between blood and muscle remained constant when the arterial oxygen tension ranged from 40 mm Hg to over 700 mm Hg, but CO shifted out of circulating blood into muscle when the arterial POT fell below 40 mm Hg. The mean myoglobin 02 tension in resting canine hamstring muscle at less than 1 y0 blood carboxyhemoglobin percent saturation and% ormal” arterial Paz (67-l 20 mm Hg) was 4.7 & (SE) 0.3 mm Hg. The mean myoglobin 02 tension decreased approximately 1.5 mm Hg as the arterial Paz was lowered from 490 to 40 mm Hg. When the arterial POT fell below 35-40 mm Hg, the mean myoglobin 02 tension was 0.5-l. 5 mm Hg. intracellular oxygen tension; muscle respiration; carbon monoxide

IN 1937 Millikan(22) described a method for the differential spectrophotometric measurement of percent reduced or oxygenated myoglobin in the soleus muscle of an anesthetized cat. In the present study we have used his concept that myoglobin may be an excellent indicator of intracellular oxygen tension and have developed a method of determining the mean oxygen tension in equilibrium with myoglobin in intact, undisturbed skeletal muscle from measurements of muscle Pco and carboxymyoglobin percent saturation. In developing this method we have studied the distribution of CO between blood and muscle as a function of arterial oxygen tensions in the anesthetized dog.