Muscle capillary permeability for [14C]inulin and [51Cr]EDTA in human forearm

Capillary permeability of [14C]inulin and [51Cr]EDTA was examined in human forearm in five healthy, subjects by indicator diffusion technique. Injections of, initially [125I]albumin and [14C]inulin, and after 30 min resting, of [125I]albumin and [51Cr]EDTA, were given in a brachial artery. During light exercise of the forearm, blood was sampled in 2-s periods from a deep cubital vein primarily draining muscles. The plasma flow rate, calculated as the dose of [125I]albumin in the injectate divided by the area under the curve for the venous concentration of 125I, was, on average, 8.5 ml min-1 100 g-1 forearm. Assuming [125I]albumin is a partially permeable tracer, a correction for extraction of albumin was performed. This gave extraction fractions of 0.107 +/- 0.015 (mean +/- SEM) for [14C]inulin and 0.377 +/- 0.033 for [51Cr]EDTA, respectively. The capillary permeability surface area product per 100 g tissue (CDC) was for [14C]inulin 0.90 +/- 0.19, and for [51Cr]EDTA 3.31 +/- 0.38 ml min-1 100 g-1 forearm. The average of the ratios of the CDC values of [51Cr]EDTA to those of [14C]inulin, 4.0 +/- 0.5, is significantly higher than the corresponding ratio between the measured free diffusion coefficients in water at 37 degrees C, 3.07 +/- 0.002 (N = 36 and 17, respectively). This indicates that there is some degree of restriction for [14C]inulin (MW 5200) relative to [51Cr]EDTA (MW 340.2) and it points to an 'equivalent pore radius estimate' of about 160 A in human muscle capillaries.