Intestinal versus hepatic contribution to circulating triglyceride levels

Fasting and postprandial overproduction of intestinally derived lipoproteins in an animal model of insulin resistance. Evidence that chronic fructose feeding in the hamster is accompanied by enhanced intestinal de novo lipogenesis and ApoB48-containing lipoprotein overproduction

Insulin-resistant states are characterized by hypertriglyceridemia, predominantly because of overproduction of hepatic very low density lipoprotein particles. The additional contribution of intestinal lipoprotein overproduction to the dyslipidemia of insulin-resistant states has not been previously appreciated. Here, we have investigated intestinal lipoprotein production in a fructose-fed hamster model of insulin resistance previously documented to have whole body and hepatic insulin resistance, and hepatic very low density lipoprotein overproduction. Chronic fructose feeding for 3 weeks induced significant oversecretion of apolipoprotein B48 (apoB48)-containing lipoproteins in the fasting state and during steady state fat feeding, based on (a) in vivo Triton WR1339 studies of apoB48 production as well as (b) ex vivo pulse-chase labeling of intestinal enterocytes from fasted and fed hamsters. ApoB48 particle overproduction was accompanied by increased intracellular apoB48 stability, enhanced lipid synthesis, higher abundance of microsomal triglyceride transfer protein mass, and a significant shift toward the secretion of larger chylomicron-like particles. ApoB48 particle overproduction was not observed with short-term fructose feeding or in vitro incubation of enterocytes with fructose. Secretion of intestinal apoB48 and triglyceride was closely linked to intestinal enterocyte de novo lipogenesis, which was up-regulated in fructose-fed hamsters. Inhibition of fatty acid synthesis by cerulenin, a fatty acid synthase inhibitor, resulted in a dose-dependent decrease in intestinal apoB48 secretion. Overall, these findings further suggest that intestinal overproduction of apoB48 lipoproteins should also be considered as a major contributor to the fasting and postprandial dyslipidemia observed in response to chronic fructose feeding and development of an insulin-resistant state.

Mechanism of hypertriglyceridemia in Dahl salt-sensitive rats, an animal model of spontaneous nephrotic syndrome

It has been reported that focal and segmental glomerulosclerosis (FSGS) with pronounced proteinuria rapidly develop in Dahl salt-sensitive hypertensive (DS) rats fed a high-salt diet. We found that even when they are fed a standard rat chow (0.3% NaCl), DS rats, especially males, exhibit marked proteinuria, hypoalbuminemia, and hypertriglyceridemia without marked hypertension at 32 to 38 weeks of age. The nephrosis was associated with spontaneously developed FSGS. We therefore investigated the mechanism of hypertriglyceridemia in nephrotic animals. Plasma triglyceride (TG) and apoprotein (apo) B levels were markedly increased in DS rats compared with Sprague-Dawley (SD) rats, and this was mainly attributable to an increase in the concentration of very-low-density lipoprotein (VLDL). The TG secretion rate estimated by the Triton WR1339 method was significantly greater in DS rats. VLDL-TGs isolated from both the DS and SD rats were endogenously radiolabeled with different isotopes, and a mixture of these was then injected into DS and SD recipients. The half-life of VLDL-TG was about three times longer in DS recipients, regardless of the source of VLDL. In SD recipients, VLDL from DS rats was cleared at a slower rate than VLDL from SD rats. The activity of lipoprotein lipase in postheparin plasma was substantially decreased in DS rats. Isoelectric focusing gel electrophoresis (IEF) showed that the ratio of apo E/C or apo C-II/C-III in VLDL was markedly decreased and the ratio of apo E or apo C to apo A1 in high-density lipoprotein (HDL) was slightly decreased in DS rats.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo determination of triglyceride secretion using radioactive glycerol in rats fed different dietary saturated fats

Our objective was to determine the relative rates of in vivo triglyceride (TG) secretion and the composition of very low density lipoproteins (VLDL) in rats fed different dietary saturated fats. Male Sprague-Dawley rats (150-200 g) were fed diets containing 16% corn oil, or 14% butterfat, 14% beef tallow, 14% olive oil, or 14% coconut oil plus 2% corn oil for 5 wk. Changes in plasma TG specific radioactivity were determined in individual, unanesthetized fasted rats after injection of 100 microCi [2-3H]glycerol. Nonlinear regression analysis using a 2-compartment model was used to determine the fractional rate constant for TG turnover in plasma. The plasma TG pool was 33-40% larger with beef tallow than with corn, olive or coconut oil feeding (p less than 0.05), and 20% larger with beef tallow than with butterfat feeding. The rate of TG secretion into plasma (mg/min/100 g body weight) was 60% higher in animals fed beef tallow than corn or coconut oil (p less than 0.05) and 26-33% higher in animals fed beef tallow than olive oil or butterfat. Differences in VLDL composition (% wt) were also noted. Our data suggest that greater TG secretion is the primary factor contributing to the larger TG pool with ingestion of beef tallow relative to butterfat, corn or coconut oil. These results suggest that different dietary saturated fats have unique effects on TG metabolism in rats.

Catabolic defect of triglyceride is associated with abnormal very-low-density lipoprotein in experimental nephrosis

Very-low-density lipoprotein (VLDL)-triglyceride (TG) kinetics were examined in puromycin aminonucleoside-induced nephrotic rats in order to establish the nature of the hypertriglyceridemia associated with this condition. Nephrotic rats had a plasma TG concentration 10-fold higher than the controls. In nephrotic rats TG secretion rate was elevated only 1.2-fold above the controls, suggesting that the catabolism of TG was also impaired. Lipolytic activities were determined in postheparin plasma (PHP) of the control and the nephrotic rats. There were no significant differences in either the activity of lipoprotein lipase (LPL) or hepatic lipase (HL). VLDL-TG was endogenously radiolabeled in donor rats with [2-3H]-glycerol. The half life (T1/2) was then determined by monitoring the clearance of plasma [3H]-VLDL-TG in normal recipient animals. The T1/2 of VLDL-TG from nephrotic rats was twice that of normal rats. The defect in VLDL-TG clearance could be partially rectified by preincubation with high-density lipoprotein (HDL) from normal rats, but not with HDL from nephrotic rats. VLDL from either nephrotic or normal rats were incubated with PHP of normal rats to assess the effectiveness of VLDL-TG as a substrate for PHP. The lipolytic rate for nephrotic VLDL was significantly lower than that for normal VLDL, suggesting that VLDL from nephrotic rats was somewhat resistant to the action of LPL and HL. When VLDL from nephrotic rats was preincubated with HDL from normal rats, the low lipolytic rate of VLDL-TG improved significantly. This was not observed when HDL from nephrotic rats was used for the preincubation. The results suggested that physical and/or chemical change of VLDL particles due to nephrosis results in a catabolic defect of VLDL-TG.

Apolipoprotein measurements in clinical biochemistry and their utility vis-a-vis conventional assays

Qualitative and quantitative measurements of apolipoproteins such as Apo CII and Apo E, as well as Apo AI and Apo B can aid in the diagnosis of specific lipoprotein metabolic defects. When used in this way, their usefulness will probably remain more or less confined to specialized research centers which have the expertise to perform such analyses and interpret the measurements. In addition, however, Apo AI and Apo B also promise to be useful for the estimation of cardiovascular risk both in individuals and in populations, and both of these apolipoprotein measurements will probably supplement rather than replace conventional lipoprotein measurements. As with the lipoproteins, the usefulness of Apo AI and Apo B measurements will depend on the accuracy and precision with which they can be measured. The current standardization efforts should lead to the development of reference methods for the measurement of total plasma levels of Apo AI and Apo B, the availability of reference materials for these apolipoproteins, and eventually, to the establishment of methods that can specifically measure Apo AI and Apo B in particular lipoprotein subfractions.

Intestinal very-low-density lipoprotein secretion in the genetically obese Zucker rat

The objectives of this study were to measure intestinal very-low-density lipoprotein (VLDL) production in obese Zucker rats and to assess an eventual effect of a high-fat diet. VLDL secretion was specifically inhibited by orotic acid, and intestinal VLDL output was measured following the Triton WR-1339 method. After a control diet, total VLDL secretion (without orotic acid) was 4.8 +/- 0.3 and 1.4 +/- 0.1 mg triacylglycerol/ml in obese and lean rats, respectively, decreasing by 30% in obese rats after fat-feeding. Intestinal VLDL production was similar in obese and lean rats fed the control diet (0.32 +/- 0.05 and 0.27 +/- 0.05 mg triacylglycerol/ml, respectively), increasing 2.5-fold after fat-feeding in both genotypes. Thus, intestine contributed 21 and 60% of total VLDL in lean but only 7 and 24% in obese rats with the control and high-fat diets, respectively. These results show that the intestine of obese Zucker rats does not contribute to their hypertriglyceridemia, suggesting that it originates solely from liver. Moreover, their intestinal VLDL production was stimulated by fat-feeding to the same extent as in lean animals.

Intestinal very low density lipoprotein secretion in rats fed various amounts of fat

1. The effect of a high-fat diet (30% fat by wt.) on intestinal very low density lipoprotein (VLDL) secretion was studied in male rats after specific inhibition of hepatic VLDL secretion by dietary orotic acid. Total VLDL secretion (from liver and intestine) was measured in animals not receiving orotic acid. 2. Fat-feeding resulted in a 32% decreased post-Triton secretion of total serum VLDL triacylglycerols as compared to a control (low fat) diet. Concomitantly, a large stimulation of post-Triton intestinal VLDL triacylglycerols secretion was measured in fat-fed rats. Thus, the major part (64%) of circulating triacylglycerols transported as VLDL originated from the intestine in these animals, leading presumably to an increased secretion of intestinal apolipoproteins. 3. Intestinal VLDL and chylomicron secretion rates increased with the amount of fat in the diet (7, 13, 20 or 30% fat by wt.). Whereas the chylomicron secretion was linearly related to the dietary fat content, the relationship between intestinal VLDL secretion and fat content of the diet was sigmoidal. The highest stimulation of intestinal VLDL formation was observed within a narrow range of dietary fat content (between 10 and 20%).

The effect of anesthesia or restraint on triacylglycerol turnover in the rat

A comparison of triacylglycerol metabolism was made among anesthetized, restrained and unanesthetized-unrestrained rats. The method utilized for the comparison was the determination of triacylglycerol turnover following the intravenous injection of [3H]glycerol. Peak appearance of triacyl[3H]glycerol was 25 min in unanesthetized-unrestrained rats but 35 and 45 min in restrained and anesthetized rats, respectively. Using serial plasma triacylglycerol determinations as an index of steady-state in all three groups, it was found that only the anesthetized and unanesthetized-unrestrained animals could be used for kinetic analysis. In these two animal preparations, apparent fractional catabolic rates were calculated and found to be lower in anesthetized (0.014 min-1) compared to unanesthetized-unrestrained animals with chronic indwelling cannulas (0.029 min-1). Apparent total catabolic rates, calculated from the plasma triacylglycerol mass and estimated plasma volume, were reduced by 50% in anesthetized animals. It is concluded that wide differences in triacylglycerol metabolism exist in these rat preparations which should be considered in the interpretation of future studies. The unanesthetized-unrestrained rat model may represent the closest approximation to the normal physiologic state.

Essential fatty acid deficiency and plasma triglyceride turnover in rats

Hepatic triglyceride secretion in essential fatty acid-deficient rats was examined by three separate techniques in an effort to resolve conflicting evidence on the question of whether essential fatty acid deficiency altered hepatic triglyceride secretion in vivo. First, plasma triglyceride turnover was measured by intravenous injection of [2-3H]glycerol trioleate. Equations of the kinetics were formulated based on a single, open pool model. Turnover rates and pool sizes of plasma triglyceride were calculated from these equations. Second, [2-3H]glycerol was injected, and apparent rate constants for plasma triglyceride secretion and clearance were calculated by kinetic analysis. Third, Triton WR-1339 was used to inhibit lipoprotein clearance from blood plasma, and rates of plasma triglyceride accumulation were measured. The results of these studies showed that the rate of hepatic triglyceride secretion was 2-3 times greater in essential fatty acid-deficient rats than in nondeficient controls. The increase in triglyceride secretion, as well as the higher level of liver triglyceride typical of essential fatty acid-deficient rats, could be caused by increased lipogenesis and increased mobilization of fatty acids from adipose tissues.

Intestinal contribution to secretion of very low density lipoproteins into plasma

To resolve the question of the magnitude of the intestine's contribution to circulating very low density lipoproteins (VLDL), measurements of intestinal, hepatic, and total VLDL--triglyceride were made on the same animals or on animals studied under comparable conditions. Animals were examined in the fasted state and during infusion of a fat-free meal. Intestinal VLDL secretion was determined through timed collections of lymph from the mesenteric lymph duct; hepatic and total VLDL secretion rates were estimated by the accumulation of plasma VLDL after injections of Triton WR 1339. Results indicate that the intestine contributes only a minor portion (11%) of the amount of triglyceride entering into the plasma compartment in the fasted state. Although intestinal triglyceride production is increased by 50% (p less than 0.01) in fed rats, the overall contribution of the intestine is not significantly altered in fed rats and represents only 14--17% of total body VLDL secretion. Thus, although intestinal VLDL secretion can be modified experimentally, its total impact on endogenous triglyceride production in normotriglyceridemic rats is small.