Fractional Extraction and Transcapillary Exchange during Continuous and Instantaneous Tracer Administration

Abstract

The relation between extraction fraction, E, measured by the multiple-tracer, instantaneous intra-arterial injection method and by single-tracer, continuous injection was studied by experiments on gastric wall, liver, and skeletal muscle of anesthetized dogs. E was calculated from ratios of diffusible to nondiffusible tracer concentrations in successive samples of venous blood. Initial extraction, E(0), was the same by instantaneous or continuous injection. E diminished with time due to back-diffusion. In liver, E(0) for water-³H, mannitol-³H, and glucose-³H remained close to unity, indicating that transcapillary transport was flow-limited. E(0) for ²²Na, ³⁶Cl, and mannitol-³H in gastric wall, and for ²²Na and ⁸⁶Rb in skeletal muscle were less than unity and decreased with increasing blood flow, indicating diffusion limitation. E for ⁸⁶Rb in skeletal muscle eventually reached a plateau level, due to the large "sink" for ⁸⁶Rb in cell K. Both E(0) and plateau E for ⁸⁶Rb in muscle increased during exercise. Permeability surface products PS were calculated from measured E's and blood flows, Q, by the relation PS = -Q ln (1 - E). Their values suggest that E(0) reflects relative capillary permeabilities to different tracers, but only for capillaries of fastest arteriovenous transit. Plateau E's, such as for ⁸⁶Rb in skeletal muscle, represent all the capillaries, but are influenced also by extracapillary barriers.