Fractional Extraction and Transcapillary Exchange during Continuous and Instantaneous Tracer Administration

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-H, mannitol-H, and glucose-H remained close to unity, indicating that transcapillary transport was flow-limited. E(0) for Na, Cl, and mannitol-H in gastric wall, and for Na and Rb in skeletal muscle were less than unity and decreased with increasing blood flow, indicating diffusion limitation. E for Rb in skeletal muscle eventually reached a plateau level, due to the large “sink” for Rb in cell K. Both E(0) and plateau E for Rb in muscle increased during exercise. Permeability surface products PS were calculated from measured Eʼs 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 Eʼs, such as for Rb in skeletal muscle, represent all the capillaries, but are influenced also by extracapillary barriers.