The permeability of the microvasculature of the perfused rat testis to small hydrophilic substances

The permeability-surface area products (PS) for sodium, Cr-EDTA, and cyanocobalamine (CoB12) have been determined in isolated perfused rat testes, using the single-passage multiple tracer technique, with albumin as the reference tracer. The validity of using albumin was established from its recovery in the perfusate leaving the testis, which was 98.73+/-0.48% of that for Cr-labeled red cells. The PS values obtained for Na, Cr-EDTA, and CoB12 were correlated with perfusate flow, both below and above levels that were equivalent to normal rates of blood flow in the testis (0.3 mL/g x min]). The values found at the highest flow rates obtained (between 2.7 and 3.5 mL/g x min]) were 2230+/-240 microL/(g x min) (n = 8) for sodium, 1460+/-140 microL/(g x min) (n = 7) for Cr-EDTA, and 850+/-80 microL/(g x min) (n = 7) for CoB12. These values are similar to those reported at equivalent flow rates for heart muscle and greater than those reported for skeletal muscle, both of which have unfenestrated capillaries similar to testis, but are less than the values for pancreas and salivary gland, which have fenestrated capillaries and are similar to most other endocrine tissues. However, the permeability coefficients for these markers in the testis (calculated using published values for the surface area of the testicular microvasculature) appear to be considerably greater than for any other tissue studied thus far. By extrapolating extraction values, either linearly or logarithmically, to obtain maximal values for PS for Cr-EDTA and CoB12, and comparing the ratio of these PS area values with the ratio of the diffusion coefficients of these molecules, it can be calculated that the equivalent pore radius for the testicular endothelium is between 5 and 6 nm, comparable to those calculated for other nonfenestrated endothelia.