Microvascular oxygen transport in rat skeletal muscle: temporal variability

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Oxygen delivery to a whole organ is closely regulated to match the metabolic needs of the tissue. But capillary blood flow at rest is characterized by considerable spatial and temporal heterogeneity suggesting that local regulation of O2 delivery does not precisely maintain O2 transport in individual capillaries. How precise is capillary O2 transport regulated? To investigate this question, we used a dual wavelength optical imaging system to measure capillary O2 transport in capillaries in extensor digitorum longus muscle of anesthetized rats for one minute periods every hour for three hours. From 5 to 15 capillaries were studied in six male Sprague-Dawley rats with normal systemic parameters (arterial blood pressure, blood gases, hemoglobin O2 saturations and hematocrit). There were no significant differences in mean capillary O2 transport parameters for each animal over the three hour period (mean ± SD; velocity μm/s: 123 ± 49, 124 ± 49, 133 ± 57; hematocrit %: 17 ± 3.5, 17 ± 5.7, 16 ± 5.2; supply rate RBC/s: 9 ± 5.2, 9 ±3.5, 10 ± 5.9; oxygen saturation %: 60 ± 9.1, 63 ± 7.3, 62 ± 14.3). However, within each animal, O2 transport parameters in individual capillaries increased or decreased from one hour to the next such that there was no significant correlation in any of these parameters over time. Although mean O2 transport for a group of capillaries was maintained over time, O2 transport was not regulated at the level of individual capillaries. Diffusional exchange among capillaries likely compensates for the heterogeneity in O2 transport in capillaries. Research supported by an operating grant from Heart and Stroke Foundation of Ontario to CGE.