Evaluation of the 'stretched pore phenomenon' in isolated rat hindquarters

In order to study the changes in capillary permeability occurring upon marked elevations in microvascular pressure, capillary filtration coefficient (CFC) and diffusion capacity (PS) for Cr-EDTA were repeatedly measured 'on-line' before and after brief periods (3 min) of large venous pressure (PV) elevations in maximally vasodilated perfused rat hindquarters. First at PV's greater than or equal to 55 mmHg, increases in CFC and Ps-Cr-EDTA were observed immediately after the pressure elevations. While the CFC increases were then always pronounced (up to 4-to 5-fold), the concomitant increases in Ps-Cr-EDTA were small (at most 30-40%). For PV greater than or equal to 55 mmHg there was a rough proportionality between PV and CFC. While PS for Cr-EDTA showed little reversibility with time upon PV normalization, CFC was usually almost completely reversed after 10-20 min. The mentioned CFC and PS increases at PV's greater than or equal to 55 mmHg were quantitatively similar to those induced by histamine-type mediators in the same preparation. It is concluded that capillary hydraulic conductivity can increase markedly but reversibly upon large PV elevations, and that this is mainly due to forceful opening of ordinarily closed 'large pores' in the microvascular membrane, rather than being caused by lesional rifts in the endothelium.