The influence of local temperature changes on the effective blood flow in the rat spinotrapezius muscle was investigated by the extension of the microtissue clearance technique originally used for frog mesentery. The clearance of water blue from microinjection sites in rat skeletal muscle at 37 C. agrees favorab1y with the clearance of other crystalloid labels from gross injection sites in skeletal muscle of mammalian forms. The data show a direct relationship between clearance rates and temperature over a range of 25 to 45 C. The µ value for clearance agrees with the µ value computed from forearm muscle blood flow measurements, at corresponding temperature ranges.

The techniques of macro- and micro-tissue clearance were used to study reactive hyperemia. Clearance of radioactive iodide from gross injection sites in muscles of rabbits and clearance of micro-injected diffusible dye from the exposed, directly observed spinotrapezius of rats gave evidence for an exact quantitative repayment of effective blood flow debt. In both types of preparation a subnormal clearance followed a thermally induced hyperemia as an apparent compensation. The exact quantitative repayment and compensation support the hypothesis that local effective blood flow is in a steady-state relation to the local concentration in tissue fluid of some agent with vasodilating properties. It is suggested that an extension of this method to studies of vasodilatation induced by drugs or other chemicals might offer an approach to identification of the normal vasodilator, and to physiological proof of its exact nature.

The effect of the synthetic peptide, bradykinin, on total and nutritional blood flow through mammalian skeletal muscle in anesthetized rats, cats, and rabbits was studied. The intra-arterial administration of the drug consistently and significantly increases total blood flow through the gastrocuemius muscle of chloralose-anesthetized cats and rabbits. A similar increase in clearance of the dye "Water Blue" from microinjection sites in the spino-trapezius muscle of the rat and clearance of iodide ions from skeletal muscle in the larger species was established. Bradykinin causes active dilatation of the nutritional circulation as well as increased total blood flow through muscle.

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