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Vascular Biology Research Centre, King's College London.

1. This study reports the results of varying the hydrostatic pressure on measurements of permeability coefficient to the low molecular weight impermeant dye carboxyfluorescein (MW = 376) in single leaky cerebral microvessels. A mathematical model, that solved the convective diffusion equations used to analyse the measurements, showed that the measurements were consistent with leakiness being due to 22 nm wide parallel-sided slits between endothelial cells. 2. Microvessels on the surface of the frog's brain were cannulated with a micropipette and perfused with an artificial cerebrospinal fluid containing the dye. Vessels were occluded with a glass microneedle and the rate of change in dye concentration in a 12 microns length section was measured using video-intensified microscopy. 3. It was found that the rate of dye loss at all points along the occluded microvessel segment could be accounted for by a model for convection and diffusion, and that changes in dye concentration at a point remote from the segment entrance can give a good measure of diffusive permeability. 4. When series of measurements were carried out on a single vessel, permeability rose over the course of 20 min. Mean permeability for all measurements was 3.01 x 10(-5) cm sec-1, n = 64 (mode, 2.0; range, 0.48-9.6). The hydrostatic pressure applied during the perfusion had no effect on the measured permeability. 5. The dye concentration along the vessel axis was measured at the steady state and was shown to respond to changes in hydrostatic perfusion pressure in a way predicted by the model. This indicates that hydrostatically driven bulk flow can be important, and thus convection may account for effects previously ascribed to vesicular transcytosis. 6. The possible anatomical basis for the porous pathway is discussed in the light of recent observations on the presence of 0.5 microns perijunctional gaps, the possibility of transendothelial channels, and the unzipping of tight junctions to leave a 22 nm wide slit.

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