Diameter changes in skeletal muscle venules during arterial pressure reduction

Previous studies in skeletal muscle have shown a substantial (>100%) increase in venous vascular resistance with arterial pressure reduction to 40 mmHg, but a microcirculatory study showed no significant venular diameter changes in the horizontal direction during this procedure. To examine the possibility of venular collapse in the vertical direction, a microscope was placed horizontally to view a vertically mounted rat spinotrapezius muscle preparation. 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 system 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 39.8 +/- 12.3 mmHg. The venular responses were not different after red blood cell aggregation was induced by Dextran 500 infusion. We conclude that diameter changes in venules over this range of arterial pressure reduction are isotropic and would likely increase venous resistance by <10%.