Contraction-by-contraction Vo₂ and computer-controlled pump perfusion as novel techniques to study skeletal muscle metabolism in situ

The purpose of this research was to develop new techniques to 1) rapidly sample venous O₂ saturation to determine contraction-by-contraction oxygen uptake (Vo₂), and 2) precisely control the rate and pattern of blood flow adjustment from one chosen steady state to another. An indwelling inline oximeter probe connected to an Oximetrix 3 meter was used to sample venous oxygen concentration ([O₂]) (via fractional saturation of Hb with O₂). Data from the Oximetrix 3 were filtered, deconvolved, and processed by a moving average second by second. Computer software and a program written in-house were used to control blood flow with a peristaltic pump. The isolated canine gastrocnemius muscle complex (GS) in situ was utilized to test these techniques. A step change in metabolic rate was elicited by stimulating GS muscles via their sciatic nerves (supramaximal voltage, 8 V; 50 Hz, 0.2-ms pulse width; train duration. 200 ms) at a rate of either 1 contraction/2 s, or 2 contractions/3 s. With arterial [O₂] maintained constant, blood flow and calculated venous [O₂] were averaged over each contraction cycle and used in the Fick equation to calculate contraction-bycontraction Vo ₂. About 5-8 times more data points were obtained with this method compared with traditional manual sampling. Software-controlled pump perfusion enabled the ability to mimic spontaneous blood flow on-kinetics (τ: 14.3 s) as well as dramatically speed (τ: 2.0 s) and slow (τ: 63.3 s) on-kinetics. These new techniques significantly improve on existing methods for mechanistically altering blood flow kinetics as well as accurately measuring muscle oxygen consumption kinetics during transitions between metabolic rates.