ANATOMICAL DIFFERENCES IN RESPONSIVENESS TO VASOCONSTRICTORS IN THE MESENTERIC VEINS FROM NORMAL AND PORTAL HYPERTENSIVE RATS

The present study evaluates the effects of pre-hepatic portal hyperten sion, induced in rats by partial portal vein ligation, on the responsi veness of rostral (proximal) and caudal (distal) rings from the mesent eric vein. The anatomical origin of the sample influenced the response to vasoconstrictors in sham-operated animals, and this pattern of rea ctivity was specifically modified in portal-ligated rats, In veins fro m sham-operated rats, contraction induced by a submaximal concentratio n of KCl (60 mM) was greater in proximal than in distal rings. Vasopre ssin and 5-hydroxytryptamine contracted mainly distal rings, methoxami ne showed a greater effect on proximal rings, and endothelin-1 and ang iotensin-II contracted vein rings independently of their anatomical or igin. In veins from portal hypertensive rats, responses to KCl (60 mM) were increased in distal rings, and all rings exhibited enhanced reac tivity to vasopressin and 5-hydroxyptyptamine as well as attenuation o f the response to methoxamine. Responses to endothelin-1 were decrease d in proximal vein rings from portal hypertensive rats whereas respons es to angiotensin-II were not influenced by the anatomical origin, Inc ubation with atropine, propranolol or indomethacin, did not modify the responses to vasopressin and 5-hydroxytryptamine in tissues from eith er sham-operated or portal hypertensive animals. Likewise, the hyporea ctivity to methoxamine and endothelin-1 in rings from portal hypertens ive rats persisted in the presence of the nitric oxide inhibitor N-G-n itro-L-arginine methyl ester, These results suggest the physiological existence of anatomical differences in the responsiveness to vasoconst rictors throughout the mesenteric vein and that changes in the respons iveness of the mesenteric vein induced by portal hypertension are spec ific for each agonist and possibly result from individual variations a t a receptor or post-receptor level.