Analysis of clearance of intra-arterially administered labels from skeletal muscle

¹ Department of Physiology, University of Southern California School of Medicine, Los Angeles, California

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.