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.
Clearance of isotope from muscle has been determined after labeling by intra-arterial infusion. If the labeling is done with normal circulation through the muscle, subsequent vasodilatation does not increase clearance. However, if muscle is labeled when dilating procedures have opened the vascular bed, subsequent dilatation or modification in effective blood flow is reflected in the clearance. Conditions for effective labeling are discussed. It is concluded that each capillary serves a limited domain of tissue fluid and that capillaries outside this domain cannot contribute to the exchange of solutes there. The findings are also consistant with the argument that blood flow in a single capillary is almost all-or-none and that, in resting muscle, nutritional blood flow is restricted to specific preferential capillary channels which remain open at all times. These findings explain some earlier paradoxes in clearance of intra-arterially administered labels.
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