The influence of portal blood upon lipid metabolism in normal and diabetic dogs and baboons.

Abstract

Complete diversion of portal blood in dogs caused sustained falls in serum cholesterol and phospholipid concentrations an declines in hepatic cholesterol and triglyceride synthesis. The hepatocytes in these canine livers were deglycogenated, and they atrophied to about half of their original size within two months. At the same time, there was evidence of increased mitoses. Ultrastructurally, the dominant change in the hepatocytes was in the rough endoplasmic reticulum which decreased in amount, underwent marked dilatation, and became depleted of ribosomes. There was also marked loss of glycogen granules, variable mitochondrial abnormalities, and widespread accumulation in the hepatocyte cytoplasm of lipid vacuoles. Bypass of intestinal venous return around the liver through a mesenteric caval shunt did not influence the serum lipid concentrations in dogs and baboons, although cholesterol synthesis was depressed in the canine livers and significant morphologic changes, including atrophy, were produced. In both species, the addition of a second stage central portacaval shunt which diverted venous return from the pancreaticogastroduosplenic area caused declines in serum cholesterol and phospholipid concentrations. After the second operation, hepatic cholesterol synthesis in the dogs was further reduced, and triglyceride synthesis was markedly depressed. The eventual ultrastructural changes were similar to those after one stage portal diversion. In other experiments on dogs, discrete regions of the liver were provided with portal perfusion from different splanchnic sources during a two month period. When the right lobes received pancreatiogastroduodenosplenic venous blood and the left lobes received intestinal venous effluent, in vivo cholesterol and triglyceride synthesis were higher in the hormone-enriched right lobes. This advantage was eliminated with pre-existing alloxan-induced diabetes or by the concomitant performance of total pancreatectomy in dogs that were treated during the ensuing two months with subcutaneously administered insulin. The nutrient-enriched left lobes had the higher lipid synthesis. In a final series of experiments, the right lobes of dogs were given the total splanchnic flow, and the left lobes were perfused with systemic venous blood by anastomosing the left portal vein to the suprarenal vena cava. The right lobar advantage in lipid synthesis could not be eliminated in this preparation with alloxan-induced diabetes or total pancreatectomy. These results indicate that a reduction of hepatic lipid synthesis is an important, although not necessarily the sole, factor in the antilipidemic influence of portacaval shunt. The effects upon synthesis and blood lipids apparently are due more to the diversion of endogenous hormones than to the bypass of intestinal nutrients. The substances in portal venous blood that subserve hepatic lipid metabolism are presumably largely the same as the hepatotropic factors which have been described before as profoundly affecting hepatic structure, function, and the capacity for regeneration. These portal blood factors are multiple and interrelated, but the single most important one seems to be insulin.