[Turnover rate of free fatty acids and triglycerides in blood in essential hyperlipemia]

Triglyceride and cholesterol metabolism in primary hypertriglyceridemia

To determine mechanisms of elevated plasma triglycerides (TG) in patients with primary hypertriglyceridemias, simultaneous studies were carried out on kinetics of very low density lipoprotein-triglycerides (VLDL-TG) and synthesis of cholesterol and bile acids. Sixteen hypertriglyceridemic patients with familial combined hyperlipidemia (FCHL) and 12 patients with poorly classified, primary hypertriglyceridemia were studied, and their results were compared to a series of normal and obese subjects previously studied in our laboratory. The mean value for transport (synthesis) of VLDL-TG in patients with FCHL was about twice normal. Although the upper normal synthesis rates overlapped with transport rates of some patients with FCHL, it appeared that the major cause of hypertriglyceridemia in FCHL was an elevated production of VLDL-TG. However, the height of the plasma TG in FCHL patients also was influenced by individual clearance capacities for VLDL-TG, and fractional clearance rates in several seemed particularly low. Synthesis rates for cholesterol and/or bile acids were high in several patients with FCHL, suggesting simultaneous overproduction of VLDL-TG and sterols; however, increased synthesis of both was not observed in all the patients. Most patients with poorly classified hypertriglyceridemia had over-production of VLDL-TG, but an apparent reduction in clearance was common. In these patients, increased synthesis of cholesterol and bile acids was infrequent. Our results indicate that abnormally high production of VLDL-TG seemed to be the major factor in causing primary hypertriglyceridemia, but that clearance capacity can play an important role in determining the the severity of the TG elevation.

Increased serum gamma-glutamyltransferase in hypertriglyceridemia: comparison with serum pseudocholinesterase

Both gamma-glutamyltransferase (gammaGT) and pseudocholinesterase (PCE) were found to be increased in hypertriglyceridemic subjects. High values of gammaGT were noted in alcoholic subjects and especially in those with either increased serum triglyceride or alanine aminotransferase higher than 16 mU/ml, while PCE was not significantly changed in alcoholic subjects. Although both enzymes were strongly correlated with the logarithm of serum triglyceride and the prebeta electrophoretic fraction, there were striking differences concerning their behavior in various hypertriglyceridemic subjects. PCE activity was high even in moderate hypertriglyceridemias but its correlation with serum triglyceride had a tendency to flatten with increasing concentration of triglyceride. However, increase of gammaGT was rather characteristic for gross hypetriglyceridemia. Short-term, triglyceride-lowering therapy was accompanied by a tendency to normalization of gammaGT, while PCE values were not influenced. An attempt was made to interpret these changes of serum-enzyme activity in hypertriglyceridemia in connection with mechanisms of lipoprotein synthesis and with the pathogeny of hyperlipemic conditions.

Fatty acids incorporation into human adipose tissue in hypertriglyceridaemia

The fatty acid and glucose incorporation into glycerides and glycerol release from adipose tissue were determined in a middle-aged population of 109 men and 41 women. 43 men and 19 women were normolipidaemic. The same analysis was also carried out in 13 male and 9 female normolipidaemic students. Needle biopsy specimens of adipose tissue were incubated in vitro in an albumin medium containing 3H-fatty acids and 14C-glucose. After two hours of incubation values for fatty acid and glucose incorporation were calculated from the incorporation of 3H-activity into the fatty acids and 14C-activity into the glycerol moiety of extracted glycerides. The mean values for fatty acid incorporation were lower in all types of hypertriglyceridaemic subjects (II B, III, IV and V) than in the normolipidaemic control subjects. In the male hypertriglyceridaemic population 36% had values for fatty acid incorporation below the 5th percentile of the normolipidaemic group and 14% had values below the lowest normal value. The rate of fatty acid incorporation was negatively correlated with the serum triglyceride concentration. This correlation remained unchanged when partial correlation was performed when the influence of body weight was eliminated. Fatty acid and glucose incorporation correlated positively. Incorporation of glucose behaved in the same way as described above for incorporation of fatty acids. Glycerol and fatty acid release was the same in the normo- and hypertriglyceridaemic groups. It is likely that the removal of plasma triglycerides from blood requires hydrolysis of triglycerides to fatty acids and the subsequent removal of the fatty acids. The hypothesis has been formulated that when the former process is normal, a defect of fatty acid removal (a low rate of fatty acid incorporation into glycerides) may be responsible for an impaired removal of plasma triglyceride-fatty acids. A low rate of fatty acid incorporation may contribute to the development of hypertriglyceridaemia, according to this hypothesis.

Human plasma triglyceride labeling after high sucrose feeding. II. Study on triglyceride kinetics and postheparin lipolytic activity

Kinetic studies of the very-low-density lipoprotein triglycerides (VLDL-TG) turnover by endogenous labeling with glycerol-2-3-H were performed in 13 patients in the postabsorptive state, first after 10-14 days on a low-sucrose high-starch diet, then again after 10-14 days of isocaloric high-sucrose low-starch diet (HSD). After HSD, a significant decrease in the fractional turnover rates of VLDL-TG was observed, as well as a modest but significant increase in its pool size, but the net turnover rates remained unchanged. Using Michaelis-Menten formulation, we have further calculated the Vmax and Km's of the removal system for VLDL-TG and found that the Vmax and Km's do not differ significantly between the two dietary periods. These results suggest that the removal mechanism for VLDL-TG has not changed after 10-14 days on the HSD, at least when the patients are studied in the postabsorptive state. Measurements of postheparin lipolytic acitivty under fed condition in 17 patients (including the 13 patients above) have shown a decrease after HSD. However, a defect in the removal of plasma-TG related to decreased activity of tissue-lipoprotein lipase in the fed state has not been conclusively uncovered by the kinetic studies performed in the postabsorptive state, and cannot contribute significantly to the expansion of VLDL-TG pool.

Diurnal patterns of triglycerides, free fatty acids, blood sugar, and insulin during carbohydrate-induction in man and their modification by nocturnal suppression of lipolysis

Previous studies have shown that carbohydrate induction of hypertriglyceridemia in normal subjects occurs at night and appears to be related to a rise of free fatty acids after diurnal feeding of high-carbohydrate formula diet. The present investigation was undertaken to observe the effect on 24-h triglyceride, free fatty acid, blood sugar, and plasma insulin profiles of inhibition of nocturnal lipolysis by glucose or nicotinic acid in normal subjects and in patients with type IV hyperlipoproteinemia. In 10 normal subjects and 10 patients with primary type IV hyperlipoproteinemia, plasma triglyceride, free fatty acid, blood sugar, and insulin levels were followed in short intervals for 24 h while a 2,400 cal, 80% carbohydrate, fat-free formula diet was given in six equal portions during the day (control experiments). This procedure was repeated in the same subjects, 10 of whom (5 normal subjects and 5 patients) received additional feedings of glucose between 2000 and 0600 h while the other 10 persons (5 normal subjects and 5 patients) were given nicotinic acid by intravenous infusion during the same time interval. Both procedures resulted in maintained lowering of free fatty acid levels over 24 h. Mitigation of carbohydrate-induced hypertriglyceridemia appeared to result from the additional glucose in normals and in patients. Nicotinic acid abolished the nocturnal rise of plasma triglyceride levels which in the control studies of normal subjects had resulted in approximate doubling of triglyceride levels in 24 h. The effectiveness of nicotinic acid in inhibiting nocturnal lipolysis and preventing carbohydrate-induction of hypertriglyceridemia might have consequences for management of endogenous hypertriglyceridemia.

Diurnal fluctuations in triglyceride, free fatty acids, and insulin during sucrose consumption and insulin infusion in man

Serial changes in circulating triglyceride, free fatty acids (FFA), insulin, and glucose have been measured in human subjects fed sucrose as the sole source of calories for 2- or 3-day periods. The sucrose was given either during the day with overnight fasting (19 subjects) or as continual 3-hour meals during the day and night (seven subjects). Insulin was infused overnight in five additional subjects on the day-feeding regimen to determine the effect on triglyceride concentration. The concentration of triglyceride increased during the study in all subjects, but there was a clear diurnal pattern in the response which was present even in the continual feeding studies. The rise in triglyceride occurred mainly overnight, and during the day there was frequently a fall in the concentration. The overnight increase was significantly less when insulin was infused. There were also diurnal fluctuations in FFA and insulin in both daytime and continual feeding regimens. The plasma FFA, like triglyceride, rose during the night and fell during the day while the insulin rose during the day and fell overnight. Separate statistical analysis of the daytime and overnight changes revealed that the changes in triglyceride were significantly but negatively correlated with changes in insulin during both periods. The changes in triglyceride and FFA were positively correlated during the day but not significantly related during the night. The data show that when sucrose is eaten for 2 or 3 days, there is a general increase in triglyceride concentration upon which are superimposed major diurnal fluctuations in the concentrations of triglyceride, insulin, and FFA. It is suggested that the highly significant inverse relationship between changes in triglyceride and insulin may be mediated through an effect of insulin on triglyceride removal.

Transport of plasma free fatty acids and triglycerides in man: a theoretical analysis

Three different multicompartmental models of free fatty acid (FFA) and very low density lipoprotein triglyceride fatty acid (VLDL-TGFA) transport in man are formulated from plasma FFA and VLDL-TGFA tracee and tracer data collected over a 24 hr interval after the injection of palmitate-(14)C. All modeling and data fitting were performed on a digital computer using the SAAM program. Structural differences in the three models relate to the position of the slowly turning over compartment required to generate the late portion of the plasma VLDL-TGFA tracer data. The positions of this slow compartment are along the hepatic pathway from FFA to VLDL-TGFA (model A) or in the distribution system of VLDL-TGFA (model B) or in the distribution system of FFA (model C). Although all three models are equally consistent with our experimental data and are supported by observations of others, each reveals inconsistency with some data obtained from the literature. Consequently, a combination model of FFA-TGFA transport, incorporating properties of models A, B, and C would be more consistent with all available data. Experiments that would help to determine the quantitative significance of each of the slow compartments in the combination model are suggested. Several other models suggesting recycling of plasma VLDL-TGFA through the plasma FFA pool, kinetic heterogencity of the plasma VLDL-TGFA pool, and contamination of plasma VLDL-TGFA radioactivity with low density lipoprotein (LDL) TGFA radioactivity were tested. The first model does not explain the late portion of the plasma VLDL-TGFA tracer data. The second and third models, while consistent with our tracee and tracer data, have steady-state implications with respect to the extent of kinetic heterogeneity and size of the LDL-TGFA contaminant that make them unlikely. Assumptions underlying other investigator's models of FFA and TGFA transport in man are reviewed within the logical framework of our models. Quantitative differences among the various models are shown by evaluating all of the models with respect to a common set of plasma FFA and VLDL-TGFA data.

Very low density lipoprotein triglyceride transport in type IV hyperlipoproteinemia and the effects of carbohydrate-rich diets

Transport of plasma-free fatty acids (FFA) and of fatty acids in triglycerides of plasma very low density lipoproteins (VLDL-TGFA) was studied in two normal subjects, five patients with type IV hyperlipoproteinemia, and two patients with type I hyperlipoproteinemia. After intravenous pulse-labeling with albumin-bound 1-palmitate-(14)C, specific radioactivity of plasma FFA and VLDL-TGFA were determined at intervals up to 24 hr. The results were analyzed using several different multicompartmental models each compatible with the experimental data. Fractional transport of VLDL-TGFA was distinctly lower (no overlap) in the type IV patients than in the control subjects, both on a usual balanced diet (40% of calories from carbohydrate) and on a high-carbohydrate diet (80% of calories). However, net or total transport of VLDL-TGFA in the type IV patients was not clearly distinguishable from that in the control subjects, there being considerable overlap on either diet. The results suggest that in this group of type IV patients the underlying defect leading to the increased pool size of VLDL-TGFA is not overproduction but a relative defect in mechanisms for removal of VLDL-TGFA. Since some of these type IV patients had only a moderate degree of hypertriglyceridemia at the time they were studied, and since it is not established that patients with the type IV phenotype constitute a biochemically homogeneous population, the present results should not be generalized. Four studies were done (in two control subjects and two type IV patients) in which the kinetic parameters in the same individual were determined on the balanced diet and on the high-carbohydrate diet. All subjects showed an increase in VLDL-TGFA pool size. Using two of the models for analysis, all showed an increase in net transport of VLDL-TGFA; using the third model, three of the four studies showed an increase in VLDL-TGFA transport. The results are compatible with the interpretation that the carbohydrate-induced increase in VLDL-TGFA, both in controls and type IV patients, is at least in part due to an increased rate of production of VLDL-TGFA. The magnitude of the increase was approximately the same in controls and patients. Thus, metabolic adjustment to a high-carbohydrate regimen in these type IV patients may not be basically different from that in normal controls; the higher levels of VLDL-TGFA reached may simply be another reflection of a defective removal mechanism. An alternative interpretation, compatible with the data, would involve both a carbohydrate-induced increase in fractional rate of release of VLDL-TGFA from liver to plasma and a decrease in fractional removal of VLDL-TGFA from plasma without increase in net production rate. The simpler hypothesis of a single primary effect on net VLDL-TGFA production from FFA seems more likely.

Splanchnic metabolism of free fatty acids and production of triglycerides of very low density lipoproteins in normotriglyceridemic and hypertriglyceridemic humans

Transport of free fatty acids from the blood into the splanchnic region and their conversion to triglycerides of very low density lipoproteins, together with estimates of splanchnic oxidation of free fatty acids to ketones and to carbon dioxide and water, have been made in the postabsorptive state in seven normolipemic subjects, six with primary endogenous hyperlipemia and one each with primary dysbetalipoproteinemia and mixed hyperlipemia. Net systemic transport of free fatty acids into the blood was the same in normolipemic and hyperlipemic groups, but a greater fraction was taken up in the splanchnic region in the latter. Transport into the blood in very low density lipoproteins of triglyceride fatty acids derived from free fatty acids was proportional and bore the same relationship to splanchnic uptake of free fatty acids in the two groups. In normolipemic subjects, near equilibration of specific activities after 4 hr infusion of palmitate-1-(14)C showed that almost all triglyceride fatty acids of very low density lipoproteins and acetoacetate were derived from free fatty acids taken up in the splanchnic region. In the hyperlipemic subjects, equilibration of free fatty acidcarbon with acetoacetate was almost complete, but not with triglyceride fatty acids, owing at least in part to increased pool size. Comparison of the rate of equilibration of triglyceride fatty acids-(14)C with rate of inflow transport from the splanchnic region, together with other data, indicated that most of the circulating triglyceride fatty acids of very low density lipoproteins in hyperlipemic subjects were also derived from free fatty acids. Although mean inflow transport of triglyceride fatty acids was greater in the hyperlipemic subjects, it correlated poorly with their concentration and it appeared that efficiency of mechanisms for extrahepatic removal must be a major determinant of the concentration of triglycerides in blood plasma of the normolipemic as well as the hyperlipemic subjects. Estimates of splanchnic respiratory quotient supported the concept that oxidation of free fatty acids accounts for almost all of splanchnic oxygen consumption in the postabsorptive state. Splanchnic oxygen consumption was greater in the hyperlipemics, but fractional oxidation of free fatty acids to ketones was higher in normolipemic subjects. Calculations of splanchnic balance indicate that a larger fraction of free fatty acids was stored in lipids of splanchnic tissues in the hyperlipemics. No differences were found between the two groups in net splanchnic transport of glucose, lactate, or glycerol.