Increased lipid synthesis by liver slice in a superfusion system following raised glucose or insulin concentration

A B S T R A C T In an attempt to define the relationship between plasma insulin and triglyceride concentrations, we have studied the effect of suppression of the postprandial insulin response upon the secretion and plasma concentration of very low density lipoprotein (VLDL)-triglycerides. Eight nondiabetic subjects with a wide range of fasting plasma triglyceride levels (100-358 mg/dl) were studied during three dietary periods: base line, high carbohydrate (80% calories), and high carbohydrate (80% calories) with a daily intravenous infusion of somatostatin (SRIF) (1.3 ,g/min) between 800 and 2,100 h. The significant increase in postprandial insulin response observed during high carbohydrate vs. base line was completely abolished during high carbohydrate-SRIF. However, plasma triglyceride levels rose in all subjects during each high carbohydrate period (with/without SRIF) vs. base line and the mean values reached during each period were the same (476±165 vs. 482±152 mg/dl, respectively). The secretion of VLDL-triglyceride into plasma was higher in four subjects, the same in two subjects, and lower in one subject during high carbohydrate-SRIF vs. high carbohydrate alone. The mean production rate of VLDL-triglyceride (mg/kg per h) was 25.6±4.9 during the high carbohydrate and 40.9±28.1 during This work was presented in part in abstract form at the

Section: Discussionsupporting

confidence: 71%

Effect of somatostatin-induced suppression of postprandial insulin response upon the hypertriglyceridemia associated with a high carbohydrate diet.

Ginsberg¹,

Jacobs²,

Le³

et al. 1982

“…Both stimulation (62)(63)(64)(65) and inhibition (66-68) of TG or VLDL secretion have been reported. Problems inherent in ascertaining the unmodified effects of insulin in vivo are numerous as counter regulatory hormones and changes to metabolite concentration may make interpretations difficult.…”

Section: Discussionmentioning

confidence: 99%

Role of insulin in lipoprotein secretion by cultured rat hepatocytes.

Patsch¹,

Franz²,

Schonfeld³

1983

173

A B S T R A C T To study the effect of insulin on lipoprotein synthesis and secretion by the liver, apoprotein and lipid levels were measured in primary rat liver cell cultures grown on fibronectin-coated dishes. Triglycerides, phospholipids, apoprotein (apo) B, apo-E, and apo-C-III3 all accumulated in culture media linearly for periods up to 20 h. During incubations, cellular triglyceride contents increased slightly, while cellular apoprotein and phospholipid contents remained constant. In the absence of insulin, rates of accumulation in media of triglycerides, apo-B, apo-C-III3, and apo-E were 2.5±0.3 Ag/mg and 33±5, 24±3, and 162±32 ng/mg cell protein per h, respectively. On gel permeation chromatography and density gradient ultracentrifugation, the majority of apoproteins in media were found to be associated with very low density lipoproteins (VLDL) and very little eluted or sedimented with albumin. Incubations in the presence of 50-800 MU/ml of insulin resulted in dose-dependent decreases of triglyceride, phospholipid, apo-B, and apo-E accumulation in the media, paralleled by increases in the cellular contents of these lipoprotein components. The inhibitory effects of insulin on secretion were reversible. Levels of apo-C-III3 and albumin were not affected by insulin. In addition to decreasing secretory rates, the proportion of apo-B, apo-E, and apo-C-III3 associated with VLDL also decreased after the addition of insulin. Concomitantly, the proportion of apo-B eluting with LDL and apo-C-1113, and apo-E eluting near albumin increased. Control experiments, in which exogenous 125I-VLDL or endogenously labeled [14C]VLDL were added to cultures, revealed that the insulin-induced differences in VLDL accumulation and the lipid association of media apoproteins were not due to differences in the processing of VLDL by cells cultured in the presence or absence of insulin. Therefore, it appears that insulin

“…The methods for measurement of VLDL secretion in whole animals also do not allow short-term effects to be studied since they require measurements to be made over several hours or days and even then there is disagreement about the theoretical models used in the interpretation of results (5). Studies of isolated liver tissues have thus proved attractive since the environment may be precisely defined and much information has been gained from studies of isolated perfused liver (6-10), perfused liver slices (11), and isolated hepatocytes in suspension (12). These methods, however, have the limitation that the viability of the isolated liver or hepatocytes can only be maintained for a short time during which they may still be under the influence of conditions prevailing in the animal at the time of their isolation.…”

mentioning

confidence: 99%

“…There has been disagreement about the role of insulin in the regulation of VLDL triglyceride secretion, some authors taking the view that it may stimulate secretion (8,11,(13)(14)(15) and others that it may be either inhibitory (10,(16)(17)(18) or without effect (6). The present series of experiments was designed primarily to resolve this issue by observing the direct effect of insulin on VLDL triglyceride secretion.…”

mentioning

confidence: 99%

Effects of Insulin and Glucose on Very Low Density Lipoprotein Triglyceride Secretion by Cultured Rat Hepatocytes

Durrington¹,

Newton²,

Weinstein³

et al. 1982

215

A B S T R A C T The effect of insulin on hepatic triglyceride synthesis and secretion is controversial. Previously, we have described a cell culture system of adult rat hepatocytes that synthesize and secrete very low density lipoprotein (VLDL) triglycerides with small and irreproducible effects of insulin on triglyceride metabolism. To study the primary effects of insulin on hepatic triglyceride metabolism a method was developed utilizing fibronectin-coated culture dishes that allowed adhesion, spreading, and maintenance of hepatocytes for 2-3 d in the absence of serum and insulin. This culture system allowed mass measurements of both cellular and secreted VLDL triglycerides for long time periods after the addition of physiological concentrations of insulin to hormone-free culture medium. In the absence of insulin and after an initial 4 h in culture, the medium was replenished and triglyceride mass was measured at the end of 18-h incubations. VLDL triglyceride accumulated in the culture medium at a linear rate over this time-course with increasing accumulation as the medium glucose concentration was raised from 2.5 to 25 mM glucose (1.77±0.24 to 3.09±0.76 yg triglyceride/mg cell protein per h). There was no apparent significant lipolysis or hepatocellular reuptake of secreted VLDL triglycerides. In the absence of insulin cellular triglyceride levels were unchanged between 3 and 24 h in culture while insulin (50-500 gU/ml) significantly increased cellular triglyceride content at all glucose concentrations tested (0-25 mM). The addition of insulin to the culture medium progressively reduced the rate of VLDL triglyceride secretion accompanied by an increase in cellular triglyceride at insulin concentrations > 50 ,U/ml. Most or all of the observed increase in cell triglyceride content could in all experiments be accounted for by the insulin-induced inhibition of VLDL secretion. Incorporation of [2-3H]glycerol into cellular and VLDL triglycerides as a function of insulin concentration was also measured. Glycerol incorporation data at 20-22 h after plating of the cells closely paralleled the insulin-induced changes in cellular and VLDL triglyceride as determined by mass analysis. The observed effects of insulin occurred at concentrations close to the physiological range and suggest that the direct hepatic effect is to suppress VLDL secretion although the net effect in vivo will clearly reflect many additional accompanying changes.