Cardiac output and muscle blood flow in exercising dogs

Average blood flow ( q ̇ ̄ ) wad determined by trapping of 15 μm radioactive microspheres in the vastus lateralis, the gastrocnemius-flexor digitorum superficialis and the triceps brachii of five 18 kg untrained mongrel dogs at rest and during graded treadmill running. Oxygen uptake (V̇ O 2 ) and cardiac output (Q̇ CO) were simultaneously determined. q ̇ ̄ leveled off in all investigated muscles at 60–100 ml·100 g −1·min −1 when V̇ O 2 was ca. 70% of peak V̇ O 2 . Q̇ CO increased linearly with V̇ O 2 up to peak V̇ O 2 . The regional blood flow (q̇ R) distribution pattern within the muscle was found to be extremely scattered around q ̇ ̄ , both at rest and at heavy exercise. q̇ R ranged from ∼ 5 to ∼ 55 ml · 100 g −1 · min −1 at rest and from ∼ 10 to ∼ 200 ml · 100 g −1 · min −1 at maximal exercise. No significant topographic pattern was observed in the q̇ R distribution of the gastrocnemius muscles which was essentially similar to that previously found for the isolated-perfused muscle preparation. The results indicate that maximal limb muscle blood flow and/or its uneven distribution may be the primary limiting factor to peak V̇ O 2 in untrained running dogs.