Synthesis and Accumulation of Triglycerides in Liver of Diabetic Rats: Effects of Insulin Treatment

Abstract

The relationship between the severity of diabetes and the hepatic accumulation of triglycerides in the streptozotocin-diabetic rat was studied. A consistent increase in hepatic triglyceride content was observed only in the ketotic diabetic state. Insulin treatment of the ketotic diabetic rat resulted in the reversal of increased plasma concentrations of glucose, free fatty acids, /3-hydroxybutyric acid, and triglycerides to those observed in nondiabetic controls. Insulin treatment of the ketotic rats, however, did not completely eliminate the accumulation of the hepatic triglycerides.To determine whether increased triglyceride synthesis was a contributory factor to the hepatic triglyceride accumulation, triglyceride synthesis was studied in vitro using nuclear-free homogenates of liver. Triglyceride synthesis increased in ketotic diabetes. Insulin treatment of the ketotic rats partially reversed the increased diglyceride synthesis observed in ketosis but had no effect on the accelerated triglyceride synthesis. The effects of ketotic diabetes and insulin treatment of the diabetic rat on the activities of two key enzymes of the triglyceride biosynthetic pathway were examined next. The maximal velocities (V max ) of liver-soluble phosphatidate phosphohydrolase and of microsomal diglyceride acyltransferase increased in ketosis. Insulin treatment of the diabetic rats reverted the activity of these two enzymes to control values.The results of our study demonstrate that hepatic triglyceride synthesis in ketotic rats accelerates at a time when hepatic triglyceride content increases, the increased synthesis of triglycerides was associated with increases in the activities of two key enzymes of triglyceride biosynthesis. There was a correlation between hepatic triglyceride content and triglyceride synthesis in control, ketotic diabetic, and insulin-treated rats. These observations suggest a physiologic role for the observed changes in the accumulation of hepatic triglycerides in ketotic diabetes. DIABETES 28:472-478, May 1979.H epatic triglycerides are known to accumulate in experimental diabetes. 1 " 3 The accumulation may result from increased synthesis of triglyceride (TG) or its decreased output from liver [as very low density lipoproteins (VLDL)] or a combination of these factors. VLDL released from liver is considered to be one measure of TG synthesis in liver, since the total output of TG by the liver is proportional to the uptake and esterification of free fatty acids (FFA). 4 -5 Hepatic TG are complexed with the appropriate apoproteins and phospholipids before released as VLDL. 6 Reports on TG synthesis, measured as release of VLDL, in liver of diabetic animals are variable. Heimberg and colleagues 4 -7 -8 observed decreased secretion of TG in perfused livers of alloxan-diabetic rats. In the pancreatectomized dog with chronic diabetes, Basso and Havel 9 noted that the secretion of VLDL-TG was depressed in response to an infusion of palmitate. Balasse, Bier, and Havel, 10 however, observed increased synthesis and re...

Regulation of hepatic triglyceride synthesis in diabetic rats.

“…Activity of this enzyme in liver cytosolic fraction was assayed using membrane-bound phosphatidate as described (1). In agreement with a recent report by Lamb et al (10), the substitution of CaCl2 for NaF in incubation mixtures for the preparation of membrane-bound phosphatidate, resulted in an increased labeling of phosphatidate fraction and reduced counts in diglyceride (DG) fraction.…”

“…A consistent 10-50-fold increase in hepatic TG content was observed in ketotic diabetic rats (1). Further studies in liver homogenates of ketotic diabetic rats demonstrated an increased substrate flux through the TG biosynthetic pathway and increases in the activity of the key enzymes of TG synthesis (1).…”

“…Previous studies in our laboratory characterized the severity of the diabetic state required for an accumulation of hepatic triglycerides (TG)1 in rats rendered diabetic with various amounts of streptozotocin (1). A consistent 10-50-fold increase in hepatic TG content was observed in ketotic diabetic rats (1).…”

“…A consistent 10-50-fold increase in hepatic TG content was observed in ketotic diabetic rats (1). Further studies in liver homogenates of ketotic diabetic rats demonstrated an increased substrate flux through the TG biosynthetic pathway and increases in the activity of the key enzymes of TG synthesis (1). An increased hepatic TG synthesis in livers of ketotic diabetic rats at a time when there was accumulation of TG suggested that the regulation of TG synthesis in these livers might have been impaired.…”

“…The sources of rats, chemicals, insulin, and methods for the induction of ketotic diabetes, insulin treatment of the diabetic rats, and assay for TG synthesis from [U-'4C]glycero-3-phosphate in liver homogenates were as described (1,2 was dissolved in a minimal volume of chloroform; 50 mM Tris-HCl buffer, pH 7.4, containing 1.5 mg purified albumin/ ml of buffer (1), was added and the suspension was sonicated at 2°C for 30 s (Biosonik IV, Bronwill Instruments, Rochester, N. Y.).…”