Diameter changes in skeletal muscle venules during arterial pressure reduction

Previous studies in skeletal muscle have shown a substantial (>100%) increa se in venous vascular resistance with arterial pressure reduction to 40 mmH g, but a microcirculatory study showed no significant venular diameter chan ges in the horizontal direction during this procedure. To examine the possi bility of venular collapse in the vertical direction, a microscope was plac ed horizontally to view a vertically mounted rat spinotrapezius muscle prep aration. We monitored the diameters of venules (mean diameter 73.8 +/- 37.0 mm, range 13-185 mu m) oriented horizontally and vertically with a video s ystem during acute arterial pressure reduction by hemorrhage. Our analysis showed small but significant (P< 0.0001) diameter reductions of 1.0 +/- 2.5 mu m and 1.8 +/- 3.1 mu m in horizontally and vertically oriented venules, respectively, upon reduction of arterial pressure from 115.0 +/- 26.3 to 3 9.8 +/- 12.3 mmHg. The venular responses were not different after red blood cell aggregation was induced by Dextran 500 infusion. We conclude that dia meter changes in venules over this range of arterial pressure reduction are isotropic and would likely increase venous resistance by <10%.