[Direct assessment of mesenteric vein compliance in the rat during pregnancy].

This study was designed for two purposes. The first was to establish an in-vitro technique for the comparative study of isolated capacitance veins during pregnancy. Secondly, to test the hypothesis that pregnancy is associated with an increase in venous compliance and the unstressed volume of the veins. Capacitance-size mesenteric veins were mounted in a pressurized myograph system equipped with a video dimension analyzing system and a servo pressure control unit, which was modified for venous studies. After equilibration, the transmural pressure was changed from 2 to 10 mmHg in 2 mmHg increments every 15 minutes. The calculated change in volume over this pressure range consisted of both a change in vessel diameter and in axial length. The veins from the nonpregnant rats exhibited a greater change in axial length than those from the pregnant rats (21% vs. 14%). Unexpectedly, the compliance (change in volume/change in pressure) of the veins from the pregnant rats was significantly less (42%) than those from the nonpregnant rats. However, the basic component of the change in vascular capacitance, the unstressed volume, was significantly greater in the veins from the pregnant rats (100%). We conclude that pregnancy causes alterations in the wall of the capacitance veins, which results in an apparent increase in volume at the expense of wall compliance. Secondly, the pressurized myograph system is a valuable tool for studying the physiology of the capacitance veins during pregnancy under rigorously controlled conditions.