Capillary Permeability in Skeletal Muscle

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Abstract
Capillary permeability in skeletal muscle was investigated by applying the single injection, external registration method to the autoperfused cat gastrocnemius preparation.51Cr‐EDTA (MW: 341.2) and57Co‐B12 (MW: 1353.5) served as simultaneously injected indicators. At a plasma flow of 7 ml/100 g×min and an assumed capillary surface area, S, of 70 cma/g the permeability coefficients Pd(51Cr‐EDTA) and Pd(57Co‐B12) were 0.74 × 10‐5cm/s and 0.45×10‐6cm/s, respectively. However, correction for the recruitment of perfused capillaries yields Pd(51C‐EDTA)‐ 1.5 × 10‐5cm/s and Pd(57Co‐B12) = 0.9 × 10‐5cm/s. The results indicate that the permeability of a muscle capillary for hydrophilic solutes is similar to that of continuous capillaries in other tissues. The Pappenheimer pore radius estimate of 30 Å, the Karnovsky 40 Å interendothelial slit width, and pinocytotic transport are defective in accounting for the results as51Cr‐EDTA and57Co‐B12 diffuse across the capillary membrane at rates proportional to their free diffusion coefficients in water (CDC(51Cr‐EDTA)/CDC(57Co‐B12) = D(51Cr‐EDTA)/D(57Co‐B12)= 1.79). The intraendothelial patent channel system of fused vesicles might be the pathway for transcapillary exchange of hydrophilic molecules as this system alone could account for the experimental results. Only 1–2 open channels per μm capillary length are necessary to account for the observed permeabilities. For both indicators CDC was a linear function of plasma flow and CDC increased 3‐3.5 fold when plasma flow increased from 4 to 52 ml/100 g× min. Assuming constant permeability coefficients, the CDC increase with plasma flow reflects an increase in capillary surface area indicating a 3‐3.5 fold recruitment of capillaries. A constant ratio of CDC(51Cr‐EDTA)/CDC(57Co‐B12) within the perfusion range indicates that51Cr‐EDTA and57Co‐B12 can be employed at plasma flow > 4 ml/100 g× min without risk of early back‐diffusion resulting in underestimation of permeability.