Absence of Restricted Diffusion in Cutaneous Capillaries

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Abstract
Capillary permeability in cutaneous tissue for57Co‐cyanocobalamin (57Co‐B12) was determined by the single injection, external registration method. The capillary diffusion capacity, CDC (the permeability‐surface area product, PS) was 2.3 ml/100 g·min. Capillary extraction was 0.48 at a plasma flow of 4.1 ml/ 100 g·min. Results were compared to51Cr‐EDTA data from a previous study with identical method and preparation. As CDC(51Cr‐EDTA)/CDC(51Co‐B12) was 1.61 and as D(51Cr‐EDTA)/D(51Co‐B12), the ratio between the free diffusion coefficients in water at 37deg;C, was 1.79 it is concluded that restricted diffusion does not occur in cutaneous tissue for57Co‐B12 as compared to51Cr‐EDTA,i.e.51Cr‐EDTA and57Co‐B12 diffuse across the capillary membrane of cutaneous tissue at rates proportional to their respective free diffusion coefficients in water. The Pappenheimer equivalent pore radius estimate of 30 Å and the Karnovsky interendothelial 40 Å slit width are both defective in explaining the experimental data. The transendothelial patent 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.