SUBATMOSPHERIC CLOSING PRESSURES IN INDIVIDUAL MICROVESSELS OF RATS AND FROGS

1. We have investigated the hypothesis that ascending vasa recta (AVR) in the rat renal medulla are able to remain open when the external pr essure is greater than the internal. 2. Individual vasa recta were can nulated in anaesthetized rats with Evans Blue albumin solution and the n occluded downstream prior to the first branchpoint. When the intra-l uminal pressure was lowered, the lumina collapsed at a mean pressure o f approximately -4.0 cmH(2)O for both AVR and descending vasa recta. 3 . The studies were extended to include microvessels from rat spinotrap ezius muscle and mesentery and frog mesentery; mean closing pressures were -3.2, -4.2 and -5.3 cmH(2)O, respectively. 4. Subatmospheric clos ing pressures may allow small differences in hydrostatic pressure alon e to drive fluid uptake into the AVR.