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_O2) 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_O2 was ca. 70% of peak V̇_O2. Q̇_CO increased linearly with V̇_O2 up to peak V̇_O2. 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_O2 in untrained running dogs.