To measure local capillary permeability to lipid-insoluble substances, we developed a microscopic tissue clearance method. It has been theoretically predicted that, when a tissue is stained with a dye by suffusing its solution around the tissue, subsequent concentration changes of the dye in the tissue due to adequate capillary flow washout takes a monoexponential time course of which decay constant is equal to the local capillary permeability surface area product (PS) per unit tissue volume. Therefore, when the capillary surface area (S) is calculated from the open capillary density in the adjacent tissue, it is possible to estimate the local permeability (P). This method was applied to the rabbit tenuissimus muscle under maximum vasodilatation, using Cr-EDTA (M.W. = 341) as a tracer. The correlation coefficient of the obtained clearance curves to the monoexponential decay was averaged to be 0.958 +/- 0.029 in 12 curves. The calculated values of Cr-EDTA permeability, 6.0 +/- 0.7 x 10(-6) cm/s, fairly well agreed with those reported for sucrose (M.W. = 342). It was concluded that this method is useful to measure local capillary permeability of small lipid-insoluble tracers.
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