Absence of restricted diffusion in adipose tissue capillaries.

Capillary permeability in adipose tissue for 57Co-cyanocobalamin (57Co-B12) was determined by the single injection, external registration method. The capillary diffusion capacity, CDC, (the permeability-surface area product, PS) was 1.1 ml/100 g-min at a capillary extraction of 0.21 and a plasma flow of 6.7 ml/100 g-min. Results were compared to 51Cr-EDTA data from a previous study with similar method and preparation. As CDC(51Cr-EDTA)/CDC(57Co-B12) was 1.81 and as D(51Cr-EDTA)/D(57Co-B12), the ratio between the free diffusion coefficients in water at 37 degrees C, was 1.79, it is concluded that restricted diffusion does not occur in cutaneous tissue for 57Co-B12 as compared to 51Cr-EDTA, i.e., 51Cr-EDTA and 57Co-B12 diffuse across the capillary membrane of adipose tissue at rates proportional to their respective free diffusion coefficients in water. The Pappenheimer equivalent pore radius estimate of 30 A and the Karnovsky interendothelial 40 A slit width are both defective in explaining the experimental data. The transendothelial channel system of fused vesicles (Simionescu, Simionescu and Palade 1975) is a possible structural equivalent for the present findings. The results support the hypothesis that capillaries of continuous type exhibit similar permeation characteristics regardless of the tissue in which they are located.