title: Prostacyclin activity in portal hypertension
Creators: name: Hamilton, George
Publisher: name: University of Glasgow
Year: 2002

description: This work was performed between 1979 and 1985 when there was great interest in the role of the recently identified prostacyclin in vascular function and disorders. The discovery of this substance with its powerful vasodilatory and platelet anti-aggregatory powers raised the hypothesis that prostacyclin might be involved in the pathogenesis of portal hypertension, its associated hyperdynamic circulation and typically catastrophic haemorrhage from bleeding oesophageal varices. Partial portal vein ligation in a rat model of portal hypertension was used because of its simplicity and absence of hepato-cellular dysfunction. This model was found to result in short lived hypertension with return to normal pressure by two weeks. Anatomical studies (using venography and corrosion casting) of the changes to the portal venous circulation after partial portal vein ligation, revealed the development of a dominant portosystemic collateral draining into the left renal vein via the left anterior lumbar vein. Ligation of this collateral at the same time as partial portal vein ligation gave a reliable model of permanent portal hypertension. Accurate measurement of prostacyclin proved to be difficult. Initially a bioassay of prostacyclin-like activity was used with success. The rat was found to produce high levels of prostacyclin well within the range of accurate measurement of this assay. Prostacyclin production was shown to increase directly with pressure increase in the portal vein. This direct relationship was confirmed in the acute model where prostacyclin production fell as the portal pressure returned to normal; in the model of chronic portal hypertension, prostacyclin production remained permanently elevated. A radioimmunoassay for 6 ketoPGFIα, the stable breakdown product of prostacyclin, was developed to allow measurement of prostacyclin in human tissue and serum samples (at this time there were no commercially available RIA kits). Initially the Wellcome antiserum was used with accurate measurement in incubated human tissue samples. These studies confirmed greater intrinsic prostacyclin activity in normal mesenteric and portal vein compared to peripheral venous tissues but failed to show any difference in tissue or plasma levels of portal hypertensive compared to normal patients. A second antiserum, the Cardeza antiserum, was then used in the radioimmunoassay. Unlike the Wellcome assay, this antiserum did not require a prostanoid extraction process. Comparison of the two assays in identical samples revealed major differences with large quantities of 6 ketoPGFIα being measured using the Wellcome antiserum with its extraction step compared to virtually none detected using the Cardeza antiserum. The extraction step was resulting in production of cross-reacting prostanoid substances giving falsely high readings. Both radioimmunoassays were abandoned at this stage because of the inaccuracy of the first, and the inability of the second to detect the low levels of 6 ketoPGFIα in human plasma. The human studies were continued using a highly specific and sensitive assay of 6 ketoPGFIα, namely gas chromatography/negative ion chemical ionisation mass spectroscopy (GC/NICIMS). At the time of this work, this methodology was complex, cumbersome, with limited access and the numbers studied were small. Very low levels of 6 ketoPGFIα were found in peripheral blood of normal and portal hypertensive patients who were not bleeding from oesophageal varices, in patients without portal hypertension, portal blood levels of prostacyclin were higher compared to peripheral levels, confirming the finding in the rat and pig that prostacyclin activity is higher in the normal portal circulation compared to peripheral vein. Significantly elevated prostacyclin production was found in both the peripheral and portal blood of portal hypertensive patients who were actively bleeding from oesophageal varices. Portal prostacyclin production was found to be significantly higher in patients with portal hypertension who were actively bleeding compared to normal patients undergoing laparotomy for other conditions. These findings in bleeding patients support the hypothesis that prostacyclin activity is increased in portal hypertension and may play a role in the severity of haemorrhage. In both the animal and human studies a clear effect of surgical intervention on increased prostacyclin production was found. These studies demonstrated for the first time increased prostacyclin production in both developing, and established portal hypertension in both the experimental animal situation and in man. High levels of prostacyclin production were found in the portal circulation of portal hypertensive patients undergoing surgery for uncontrolled variceal bleeding.