Hemodynamic studies in long- and short-term portal hypertensive rats: the relation to systemic glucagon levels

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.

Effects of glucagon on systemic and splanchnic circulation in conscious rats with biliary cirrhosis

To elucidate the relationship between the haemodynamic changes and glucagon in cirrhosis, we infused physiologic and supraphysiologic doses of this hormone in conscious rats with portal hypertension due to biliary cirrhosis. Cardiac output and splanchnic organ blood flows were measured by the radioactive microsphere method before and 30 min after glucagon infusion at doses of 2, 5 and 10 ng/min. Serum glucagon increased from a basal level of 92 +/- 17 pg/ml (mean +/- S.E.) to 399 +/- 89, 1151 +/- 136 and 2064 +/- 328 pg/ml, respectively, in sham-operated rats, and from 743 +/- 75 pg/ml to 1497 +/- 197, 1583 +/- 356 and 2957 +/- 649 pg/ml, respectively, in cirrhotic animals at 2, 5 and 10 ng/min doses. In both groups, cardiac output did not change after glucagon infusion at 2 and 5 ng/min doses, suggesting that factors other than glucagon are primarily responsible for the systemic hyperdynamic circulation in cirrhosis. Portal tributary blood flow increased significantly after glucagon infusion in sham-operated rats by 34 and 65% at doses of 5 ng/ml and 10 ng/ml, respectively, and in cirrhotic rats by 29% at a dose of 10 ng/ml. However, portal tributary blood flow did not change after glucagon infusion at the physiologic dose of 2 ng/min. This study shows that glucagon infused at a physiologic dose does not increase splanchnic blood flow, although it increases portal tributary blood flow at supraphysiologic doses. The discrepancy between blood glucagon levels and splanchnic haemodynamic responses suggests that glucagon plays only a minor role and that other factors are primarily responsible for the hyperdynamic state of the splanchnic circulation in rats with biliary cirrhosis.

Portal hypertension ameliorates arterial hypertension in spontaneously hypertensive rats

Systemic hemodynamic effects of portal hypertension in arterial hypertension and their relationship to serum bile acid levels were investigated using spontaneously hypertensive rats 2 and 15 weeks after partial portal vein ligation (SHR-PVL) or sham operation (SHR-SH) and normotensive controls. Mean arterial pressure in SHR-PVL at 2 weeks was decreased to normal due to a decrease in peripheral resistance. Mean arterial pressure and peripheral resistance in SHR-PVL at 15 weeks did not differ from SHR-SH. Resolution of this arterial hypotensive effect and systemic hyperdynamic circulation was associated with decreased portal-systemic shunting. Bile acid levels were increased in both SHR-PVL groups. These results suggest that an endogenous circulating vasodilator(s) associated with portal hypertension ameliorates the systemic vasoconstriction in SHR. Bile acids, while not direct mediators of these hemodynamic events, may be prototypic of this vasodilator. This arterial hypertensive model may aid further investigation of the mechanisms contributing to the hyperdynamic state in portal hypertension.

Role of prostacyclin in the splanchnic hyperemia contributing to portal hypertension

To determine the possible role of prostacyclin (PGI2) as a mediator of the splanchnic hyperemia seen with portal hypertension, the portal and mesenteric hemodynamics in normal and portal hypertensive rabbits were studied before and after cyclo-oxygenase blockade. Three weeks after partial portal vein ligation, splenic pulp pressure was elevated from 4.3 +/- 0.9 to 9.8 +/- 0.8 mmHg (p less than 0.01). Mesenteric blood flow increased from 77.0 +/- 4.7 ml.min-1.100 g-1 to 99.1 +/- 5.19 ml/min-1/100 g-1. Mesenteric vascular resistance fell from 0.82 +/- 0.6 mmHg/ml-1/min-1 to 0.49 +/- 0.07 mmHg/ml-1/min-1 (p less than 0.01). These hemodynamic changes were associated with a 27.3 +/- 0.2% rise in systemic arterial levels of PGI2 (p less than 0.01) and were substantially ameliorated by cyclo-oxygenase blockade with indomethacin. The effects of indomethacin blockade were reversed by exogenous PGI2. Moreover, in normotensive rabbits, infusion of PGI2 reproduced the splanchnic hyperemia and caused a very small but significant increase in portosystemic shunting. These findings support the previously proposed concept that splanchnic hyperemia may contribute to the maintenance of chronic portal hypertension. Furthermore, they suggest that this effect may be partially mediated by splanchnic PGI2 production.

Pathophysiology and epidemiology of portal hypertension

Changes in portal pressure are regulated by changes in hepatic vascular resistance, which is normally under neurohumoral control, and portal tributary blood flow. Two theories on the pathophysiology of portal hypertension have been proposed: the 'backward flow' theory, in which portal hypertension is attributable to increased resistance to portal venous flow, and the 'forward flow' theory, in which increased splanchnic blood flow maintains portal hypertension despite extreme portal-systemic shunting. The sinusoidal abnormalities caused by an accumulation of collagen in the perisinusoidal space of Disse may induce increased resistance to blood flow in various pathological conditions of the liver. Non-cirrhotic portal hypertension results from not only relatively uncommon disorders prevalent mainly in Asia and tropical countries, but also from acute and chronic phases of relatively common liver diseases. Systemic hyperdynamic circulation, characterised by an increased cardiac output and a reduced peripheral vascular resistance, and splanchnic hyperaemia may develop as consequences of portal hypertension. Although the mechanisms of these changes are not clearly understood, portal-systemic shunting as well as some vasoactive substances, including prostaglandins, may be involved. The erosive and eruptive mechanisms are the two potential explanations for variceal bleeding. In the latter, pressure should not be viewed in isolation and other additive factors such as variceal size may be involved. Several new techniques of measuring variceal pressure and blood flow may improve understanding of the actual pathophysiology of variceal bleeding. Renal haemodynamic alterations secondary to the systemic circulatory changes produced by portal hypertension may occur. The geographical pattern of prevalence in disorders associated with portal hypertension is briefly described in this paper.

Reduction of intrahepatic vascular space in the pathogenesis of portal hypertension. In vitro and in vivo studies in the rat

To elucidate a possible role for reduction of intrahepatic vascular space in the pathogenesis of portal hypertension, we studied a partially hepatectomized rat model in vitro and in vivo. The in vitro study used livers from normal, 1/3-hepatectomized, and 2/3-hepatectomized rats, and rats with cirrhosis caused by chronic bile duct ligation, for isolated, perfused flow-pressure plotting. Resistance to perfusion increased such that significant differences were found between all groups except the last two, which showed similar resistances. The in vivo study measured splanchnic blood flow by radioactive microspheres and portal pressure in anesthetized sham-operated and 1/3- and 2/3-hepatectomized rats. Although absolute portal tributary blood flows did not change, portal flow per gram of remnant liver showed significant increases: 1.57 +/- 0.32 ml/min X g liver, 2.52 +/- 0.60 ml/min X g liver, p less than 0.01; 3.48 +/- 1.04 ml/min X g liver, p less than 0.01, respectively. Although intrahepatic resistance increased significantly only in the 2/3-hepatectomized group, portal pressures increased significantly in both groups of hepatectomized rats: normal, 7.5 +/- 1.1 mmHg; 1/3-hepatectomized, 9.4 +/- 1.1 mmHg; and 2/3-hepatectomized, 11.1 +/- 1.2 mmHg. Thus, decreased intrahepatic vascular space caused by resection and hepatocellular hypertrophy leads to portal hypertension, thereby suggesting that this reduction in space may be the pathogenic factor common to a number of different theories of portal hypertension.