To clarify the transport of O2 across the microvessels in skeletal muscle, we designed an intravital laser microscope that utilizes a phosphorescence quenching technique to determine both the microvascular and tissue Po2. After we injected the phosphorescent probe into systemic blood, phosphorescence excited by a N2-dye pulse laser was detected with a photomultiplier over a 10 μm in diameter area. In vitro and in vivo calibrations confirmed that the present method is accurate for Po2 measurements in the range of 7-90 Torr (r = 0.958) and has a rapid response time. This method was then used to measure the Po2 of microvessels with different diameters (40-130 μm) and of interstitial spaces in rat cremaster muscle. These measurements showed a significant drop in Po2 in the arterioles after branching (from 74.6 to 46.6 Torr) and the presence of a large Po2 gradient at the blood-tissue interface of arterioles (15-20 Torr). These findings suggest that capillaries are not the sole source of oxygen supply to surrounding tissue.
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