Reflex control of mesenteric vein diameter and pressure in situ in rabbits.

We have directly demonstrated that the mesenteric veins actively constrict and dilate in response to reflex stimuli in the in situ preparation. A segment of small intestine of 23 chloralose-anesthetized rabbits was exposed in a specially designed bath. Small vein diameter and intravenous pressure (575-1,375 microns and 6.2-11.4 mmHg at rest, respectively) of the mesentery were measured continuously at the same site with the use of videomicrometer and micropressure systems during celiac ganglion stimulation (CGS), bilateral carotid occlusion (BCO), and aortic nerve stimulation (ANS). Innervation to the mesenteric vein was evident by a frequency-dependent venoconstriction in response to CGS, which was completely abolished by local application of tetrodotoxin. Vein diameter and intravenous pressure changed from an average of 1,042 +/- 32 to 1,003 +/- 32 microns (-3.8 +/- 0.3%, P less than 0.01) and from 8.1 +/- 0.3 to 9.0 +/- 0.3 mmHg (10.6 +/- 0.8%, P less than 0.01), respectively, during BCO. During ANS, vein diameter increased from 990 +/- 32 to 1,012 +/- 31 microns (2.4 +/- 0.2%, P less than 0.01), and intravenous pressure decreased from 8.5 +/- 0.3 to 7.9 +/- 0.2 mmHg (-6.1 +/- 0.6%, P less than 0.01), respectively. Vein diameter changes of -3.8 and 2.4% corresponded to volume changes of -7.6 and 4.9%, respectively. During BCO, vein diameter decreased while intravenous pressure increased, whereas during ANS vein diameter increased while intravenous pressure decreased. These results indicate that the reflex change in vein diameter is due to an active response.