Analysis of oxygen and NADH fluorescence to determine critical O2 (O ₂) tension in skeletal muscle in vivo and in vitro

When muscle work and metabolism increase, blood flow increases to maintain supply of O ₂ and other substrates. To explain matching of blood flow to metabolism, it has been proposed that vasodilating metabolic end products such as adenosine, produced when mitochondria in muscle become O ₂ limited, act at arterioles to increase blood flow. However, blood flow is regulated at O ₂ tensions at least 10 fold higher than the Km of cytochrome oxidase for O ₂ in isolated mitochondria. Mitochondrial metabolic state (NADH fluorescence) and O ₂ tension (porphyrin phosphorescence) were measured in exteriorized spinotrapezius muscle and isolated myocytes to determine if the O ₂ sensitivity for isolated mitochondria is different than that of mitochondria in intact cells or tissue In isolated cells NADH fluorescence was verified as a good indicator of mitochondrial metabolic state which, similar to isolated mitochondria, became inhibited near 1 mmHg O ₂. In intact muscle, the O ₂ sensitivity was slightly higher than in isolated cells (2.4 mmHg, n=12). However, mean PO ₂ measured in muscle is approximately 20 mmHg which is too high for proposed metabolic feedback mechanism to operate. These findings suggest that vasodilator products of anaerobic metabolism are not involved in blood flow regulation under normal working conditions, but only in highly O ₂ deficient or non-oxidative tissue regions.