Comparative Studies of Capillary Permeability in Brain and Muscle

Abstract

Abstract The permeability of the capillary wall in the brain was compared with that in muscle by means of the ratio between outflow concentrations (relative to inflow concentrations) after single i.a. injections of various combinations of two radioactive tracers. A ratio above 1 means that the solute in the denominator left the capillaries faster than the other tracer, v ice versa. The following outflow ratios were obtained: Hind limb : Glucose/K + and 3‐O‐methyl‐glucose/K + : above 1. Brain : Glucose/K + and 3‐O‐methyl‐glucose/K + : below 1. The hind limb results show a faster transcapillary transport of potassium than of the sugars—in agreement with the predictions of the Pappenheimer pore model for muscle capillaries. In brain the sugars are transported faster across the capillary wall than tracer potassium. This finding cannot be explained by diffusion through a common interendothelial diffusion pathway (pore). but signify that the sugars are transported through the capillary endothelial cells in brain—a route not available to potassium. The Pappenheimer pore model only applies to the brain capillaries as a limiting case with a pore permeability close to zero, even for small solutes. Thus, the permeability characteristics of the brain capillaries are in closer agreement with those of mono‐layered epithelia than with the Pappenheimer pore model. It is concluded that important blood‐brain barrier functions are carried out in the cerebral capillary walls.