The relationship of blood flow to myoglobin, capillary density, and twitch characteristics in red and white skeletal muscle in cat.

1. The fractional distribution of blood flow was measured by the isotope dilution method and absolute blood flow estimated in twenty-four muscles of chronically prepared, unanaesthetized cats.2. In the quiet, alert cat the estimated mean muscle blood flow ranged from 5.9 to 39.3 ml./min 100 g muscle. Flow was three times greater in grouped red than white limb muscles.3. The myoglobin concentration ranged from 0.88 to 3.91 mg/g muscle. The myoglobin concentration of grouped red exceeded that of white muscle and varied in each muscle directly and linearly with blood flow with a high correlation (P < 0.001).4. Capillary density was estimated by determination of alkaline phosphatase activity. Alkaline phosphatase activity was significantly greater in red than white muscles and varied directly with fractional blood flow in these same muscles with a high correlation (P < 0.001).5. Contraction time was generally two to three times longer in red than white muscle. There was a direct and highly correlated relationship (P < 0.001) between blood flow and contraction time.6. The direct linear relationship between muscle blood flow and myoglobin concentration, capillary density, and contraction time characteristic of quiet wakefulness was lost during rapid eye movement sleep or excitement produced by electrical stimulation of the hypothalamus as a consequence of selective changes in flow in red and white skeletal muscle.7. The type of metabolism of any skeletal muscle is adapted to its activity during quiet, alert behaviour, rather than during sleep or excitement.