Changes in transcapillary exchange induced by perfusion fixation with glutaraldehyde, followed by measurements of capillary filtration coefficient, diffusion capacity and albumin clearance.

The isolated, maximally vasodilated rat hindquarter preparation was used to determine how fixation of the vascular bed (glutaraldehyde, 2.5% in 0.05 M Na-cacodylate with dextran 40 g X 1-I) affected transcapillary filtration and diffusion events, as well as macromolecular transfer, measured as transport of water (CFC), of small solutes (PS) and of 125I-albumin. Vascular conductance to flow was decreased by 52% after fixation. The three capillary permeability-surface area parameters were reduced almost equally to 50% of control after fixation, i.e. the relative flux rates of water, small and large molecules were similar in the living and in the fixed vascular bed. As this fall in transendothelial exchange was larger than the calculated degree of wall shrinkage of the resistance vessels, it could not be ascribed simply to a uniform shrinkage which also involved the capillaries. Rather, the fixation appears to have induced closure of entire capillaries. As a consequence, the surface area for exchange is reduced, with no apparent changes in membrane 'permselectivity'.