Localization and Measurement of Microvascular and Microcirculatory Responses to Venous Pressure Elevation in the Rat

The effect of increase in regional draining venous pressure on the response of mesoappendix microvasculature and microcirculation was studied by direct microscopic methods in vivo. In 9 out of 21 experiments in pentobarbital anesthetized rats, elevation of ileo-colic vein pressure (13.0 ± 1.3 mm Hg; mean and SE) above resting levels (9.9 ± 2.2 mm Hg) resulted in persistent lumen expansion of small venular microvessels (18-68 µm, i.d.) and evoked a vasoconstrictor response of consecutive segments of pre-capillary vessels and the sphincter. The vasoconstricor response was stimulus-dependent and reversible. The lumen reductions in percent for 9 arterioles (21.0 ± 4.37 µm, i.d.) 9 metarterioles (9.3 ± 1.57 µm, i.d.), and 5 precapillary sphincters (5.7 ± 1.23 µm, i.d.) were: 17.9 ± 6.60, 43.6 ± 4.20, and 96.0 ± 3.81, respectively. In none of the experiments did the response propagate to larger arteriolar segments ( > 35–38 µm, i.d.) in the tissue under observation. These microscopic studies confirmed the presence of a venous-pre-capillary and sphincter constrictor response in splanchnic vasculature in the rat.