Insulin resistance in uremia. Characterization of insulin action, binding, and processing in isolated hepatocytes from chronic uremic rats.

Abstract

A B S T R A C T We have developed a model in the rat that leads to a predictable degree of severe uremia to study the role of the liver in the insulin-resistant state of uremia. The uremic animals were euglycemic and had increased serum immunoreactive insulin when compared with their pair-fed controls. Insulin action, binding, internalization, and degradation were characterized in freshly isolated hepatocytes from uremic animals, sham-operated pair-fed, and ad lib.-fed controls.The basal rate of aminoisobutyric acid (AIB) uptake was increased in hepatocytes from both uremic and pair-fed control rats. However, while hepatocytes from uremic animals were refractory to insulin with regard to AIB uptake, there was no significant difference in the absolute increment above basal AIB uptake by hepatocytes from pair-fed and fed ad lib. animals at any insulin concentration studied.125I-Insulin binding at 240C was higher in hepatocytes from uremic rats at every insulin concentration studied when compared with fed ad lib. controls. The time course of '251-insulin binding to the cell and to the fractions that were membrane bound or internalized were studied at 370C. An increase in membranebound '251-insulin at 370C was present also in hepatocytes from uremic animals. The same fraction of membrane-bound 1251-insulin was internalized in hepatocytes from all groups of animals. nalization was studied at 370C by gel chromatography. There was a delayed and decreased rate of 125I-insulin degradation in hepatocytes from uremic rats in the three compartments.We conclude: (a) In chronic uremia the liver is refractory to insulin with regard to AIB uptake. (b) Insulin resistance in uremic rat liver is not due to defects in insulin binding or internalization. (c) Despite the high level of circulating immunoreactive insulin, hepatocytes from uremic rats did not show the expected "down regulation" of their insulin receptors or an increased rate of insulin degradation. These studies further emphasize the primary role of postbinding events in the regulation of insulin binding and degradation. The mechanism as to how the coordinated steps of insulin metabolism in the liver are disrupted in a pathological state is presently unknown. INTRODUCTIONInsulin resistance is widely recognized in patients with chronic renal failure (1). Peripheral insulin insensitivity in uremia is well accepted, but the role of the liver in insulin action, binding, and processing (internalization and degradation) remains controversial or unexplored (2, 3). This is an important problem to define since the liver is a major target organ for insulin action and the main organ for insulin metabolism and degradation (4).In order to study hepatic insulin metabolism in uremia we have developed a model of chronic renal failure in the rat that leads to a predictable degree of severe uremia. In the experimental design, specific consideration was given to the nutritional status of the animals. Two sham-operated control groups, ad lib.-fed and pair-fed animals were included. This al...

Insulin resistance in uremia. Characterization of lipid metabolism in freshly isolated and primary cultures of hepatocytes from chronic uremic rats.

“…The uremic animals have elevated serum triglyceride and normal serum cholesterol when compared with their pair-fed, age-, and weight-matched controls. Also, these uremic rats are hyperinsulinemic and normoglycemic (8). These characteristics of experimental uremic rats are similar to those found in uremic humans (30,31), thus making it a suitable animal model to investigate these problems.…”

“…We have recently demonstrated that the liver in uremia is insulin resistant with regard to amino acid transport, which is not due to defects either in insulin binding or its internalization (8). DeFronzo et al (7) have demonstrated a normal suppression of hepatic glucose production by insulin in uremic patients.…”

“…3H20 ( Experimental model of chronic uremia. We have recently described the uremic rat model used in this study (8). Briefly, male Sprague Dawley rats weighing 200 g were anesthetized with ether.…”

“…Although peripheral insulin insensitivity in uremia is well accepted, insulin action in the liver is unsettled. DeFronzo an uremic rat model that the liver is resistant to insulin with regard to amino acid transport (8). If the uremic liver were insulin sensitive to stimulation of lipid synthesis, it could be expected that the elevated basal immunoreactive insulin concentration would increase endogenous triglyceride production.…”

“…Hepatocytes were isolated from a rat model that leads to a predictable degree of severe uremia (8). In the experimental design, specific consideration was given to the nutritional status of the animals.…”