The Effect of Hyperosmolality on the Permeability and Structure of the Capillaries of the Isolated Rete Mirabile of the Eel

Capillary permeability estimates from osmotic transient experiments generally have been found to be higher than those obtained from tracer experiments. The question which arises is whether hyperosmolar infusions affect capillary permeability. This was investigated in the rete mirabile of the eel, an organ made up of alternately disposed arterial and venous capillaries. Permeabilities were determined from the steady arterial to venous capillary transfer of labeled albumin and labeled urea (large and small molecules traversing a pathway of limited area) and labeled water (which traverses the whole surface) during countercurrent perfusion with Krebs-Ringer bicarbonate buffer containing bovine serum albumin, both with and without the addition of 350 mOsm/liter sucrose. During bipolar hyperosmolar perfueion, the permeability to labeled albumin and urea increased by two-thirds, and that to labeled water decreased by one-third. Hyperosmolar perfusion of arteriolar vessels only resulted in even larger increases in solute permeabilities. Ultrastructural examination complemented by morphometry showed that, with bipolar hyperosmolar perfusion, the volumes of arterial and venous capillary endothelial cells and pericytes decreased, and that extracapillary intercellular edema accumulated; and that the volume density of the vesicular and tubular system in the arterial capillary endothelial cells, the principal barrier in the system, was increased by two-thirds. The protein A-gold method demonstrated that albumin molecules localized predominantly to the vesicular and tubular system of the arterial endothelial cells and not particularly to the intercellular junctions. The functional data indicate that, in these experiments, the hyperosmolar perfusion increased capillary permeability and the morphological data suggest that the parallel increase in the volumes of the vesicular and tubular system in some fashion mediated the increase in permeability.