Blood flow, $$P_{O_2 } $$ and pH during progressive working contractions in a whole muscle group

Alterations in blood flow during progressive working contractions were examined to elucidate their relation to work rate in a predominantly glycolytic muscle group, i.e., m. gastrocnemius and m. plantaris, in rabbits anesthetized with urethane and chloralose. In one series of animals, the sciatic nerve was stimulated to induce plantar flexions of constant length at 2, 5 and 8% of an afterload at which only isometric tension could be developed. Another series was exercised at 30 and 50% of this value, and a third group served as non-exercised controls. Each experimental session consisted of a series of 5 min nonexercise periods followed by 6 min exercise periods, and a 10 min post-exercise period. Femoral venous blood was obtained just before the first exercise period, during the final minute of each exercise period, and 10 min after the final exercise period. The composition of venous blood samples from control animals did not change during the experimental session. Blood flow in the exercising limb increased at the lowest workload, and attained a maximum flow rate at the 5% workload. Blood gases were altered to a similar extent at all afterloads, averaging: \(P_{O_2 } = 4.0 \pm 0.2\) kPa and \(P_{CO_2 = 7.5 \pm 0.3}\) kPa. pH, in contrast, was lower at the heaviest afterloads (X=7.144±0.03) compared to the lighter afterloads (X=7.245±0.03). The blood flow and pH patterns are consistent with the glycolytic fiber type composition of this muscle group. Venous \(P_{O_2 } \) indicates that O₂ delivery was adequate, even at the highest afterload. Thus, the pattern of blood flow obtained in larger vessels may reflect primarily the fiber type composition of the various active muscles, and the net composite O₂ requirements.