Splanchnic and hepatic triglyceride secretion during hypercaloric intravenous glucose infusion in conscious swine.

We have validated a radiochemical technique for measuring the rate of secretion of plasma triglycerides from the liver and/or splanchnic region during the consumption of glucose under isotopic steady-state conditions. Values obtained with this technique correlated closely with those based on transhepatic or transsplanchnic chemical gradients (r = 0.95). Likewise, values for secretion of triglycerides obtained with the radiochemical technique correlated closely with those obtained for extrahepatic or extrasplanchnic triglyceride clearance. Values for mean net splanchnic and hepatic secretion of plasma triglyceride fatty acids, transported essentially in very low density lipoproteins, were 1.9 and 2.0 mumoles/min.kg body wt0.75, respectively, about one-half of the rate of transport of free fatty acids. However, the fraction of triglyceride fatty acids of plasma very low density lipoproteins that was derived from plasma free fatty acids averaged 9% and that derived from glucose, though increasing with time, reached only 2% after constant intravenous infusion of radioglucose for 5 hr. Porcine hepatic secretion of plasma triglycerides is large in the glucose-fed state, and the secreted triglyceride fatty acids evidently are derived from stored fat or glycon.     

Acute inhibition of lipolysis does not affect postprandial suppression of endogenous glucose production

To test the hypothesis that intrahepatic availability of fatty acid could modify the rate of suppression of endogenous glucose production (EGP), acipimox or placebo was administered before and during a test meal. We used a modified isotopic methodology to measure EGP in 11 healthy subjects, and (1)H magnetic resonance spectroscopic measurement of hepatic triglyceride stores was also undertaken. Acipimox suppressed plasma free fatty acids markedly before the meal (0.05 +/- 0.01 mmol/l at -10 min, P = 0) and throughout the postprandial period (0.03 +/- 0.01 mmol/l at 150 min). Mean peak plasma glucose was significantly lower after the meal on acipimox days (8.9 +/- 0.4 vs. 10.1 +/- 0.5 mmol/l, P < 0.01), as was mean peak serum insulin (653.1 +/- 99.9 vs. 909 +/- 118 pmol/l, P < 0.01). Fasting EGP was similar (11.15 +/- 0.58 micromol.kg(-1).min(-1) placebo vs. 11.17 +/- 0.89 mg.kg(-1).min(-1) acipimox). The rate of suppression of EGP after the meal was almost identical on the 2 test days (4.36 +/- 1.52 vs. 3.69 +/- 1.21 micromol.kg(-1).min(-1) at 40 min). There was a significant negative correlation between the acipimox-induced decrease in peak plasma glucose and liver triglyceride content (r = -0.827, P = 0.002), suggesting that, when levels of liver fat were low, inhibition of lipolysis was able to affect glucose homeostasis. Acute pharmacological sequestration of fatty acids in triglyceride stores improves postprandial glucose homeostasis without effect on the immediate postprandial suppression of EGP.     

Effects of glucose ingestion on postprandial lipemia and triglyceride clearance in humans

To determine whether the metabolism of diet-derived triglycerides (TG) is acutely regulated by the consumption of insulinogenic carbohydrates, we measured the effects of glucose ingestion on oral and intravenous fat tolerance, and on serum triglyceride concentrations obtained during duodenal fat perfusion. Postprandial lipemia was diminished by the ingestion of 50 g (148 +/- 121 mg.dl-1 x 7 h-1 vs 192 +/- 124 mg.dl-1 x 7 h-1, P less than 0.05) and 100 g (104 +/- 106 mg.dl-1 x 7 h-1 vs 171 +/- 104 mg.dl-1 x 7 h-1, P less than 0.05) glucose. Peak postprandial TG concentrations occurred later after meals containing glucose and fat than after meals containing fat alone. This effect could be reproduced when an iso-osmotic quantity of urea was substituted for glucose in the test meal. Starch ingestion had no discernible effect on postprandial lipemia. Intravenous fat tolerance was similar before (4.9 +/- 1.2%.min-1) and 2 h (4.4 +/- 1.3%.min-1) and 4 h (4.8 +/- 1.5%.min-1) after 50 g glucose ingestion. During duodenal fat perfusion, glucose ingestion caused a progressive decrease in plasma triglyceride concentrations. These data suggest that glucose ingestion diminishes postprandial lipemia in a dose-dependent manner, but that this effect is not due to increased clearance of triglyceride from the circulation. The hypotriglyceridemic effects of glucose appear to reflect delayed gastric emptying and decreased hepatic secretion of triglyceride.     

Relationship between carbohydrate-induced hypertriglyceridemia and fatty acid synthesis in lean and obese subjects

We previously reported that a eucaloric, low fat, liquid formula diet enriched in simple carbohydrate markedly increased the synthesis of fatty acids in lean volunteers. To examine the diet sensitivity of obese subjects, 7 obese and 12 lean volunteers were given two eucaloric low fat solid food diets enriched in simple sugars for 2 weeks each in a random-order, cross-over design (10% fat, 75% carbohydrate vs. 30% fat, 55% carbohydrate, ratio of sugar to starch 60:40). The fatty acid compositions of both diets were matched to the composition of each subject's adipose tissue and fatty acid synthesis measured by the method of linoleate dilution in plasma VLDL triglyceride. In all subjects, the maximum % de novo synthesized fatty acids in VLDL triglyceride 3;-9 h after the last meal was higher on the 10% versus the 30% fat diet. There was no significant difference between the dietary effects on lean (43+/-13 vs. 12+/-13%) and obese (37+/-15 vs. 6+/-6%) subjects, despite 2-fold elevated levels of insulin and reduced glucagon levels in the obese. Similar results were obtained for de novo palmitate synthesis in VLDL triglyceride measured by mass isotopomer distribution analysis after infusion of [(13)C]acetate. On the 10% fat diet, plasma triglycerides (fasting and 24 h) were increased and correlated with fatty acid synthesis. Triglycerides were higher when fatty acid synthesis was constantly elevated rather than having diurnal variation.Thus, eucaloric, solid food diets which are very low in fat and high in simple sugars markedly stimulate fatty acid synthesis from carbohydrate, and plasma triglycerides increase in proportion to the amount of fatty acid synthesis. However, this dietary effect is not related to body mass index, insulin, or glucagon levels.     

Effect of medium-chain triglycerides and long-chain triglycerides on plasma pancreatic polypeptide secretion in man

Since it has been shown that stimulation of pancreatic enzyme secretion by triglycerides is dependent on the chain length of the fatty acids, we have studied whether the secretion of pancreatic polypeptide (PP) in response to triglycerides is also related to the chain length of the fatty acids. Therefore, the effect of equimolar amounts (60 mmol) of medium-chain triglycerides (MCT) and long-chain triglycerides (LCT) on plasma PP was studied in 6 normal subjects. In the control study the subjects ingested 60 ml of 0.15 mol/l saline. Ingestion of LCT resulted in significant increases in plasma PP from 33 +/- 7 to 55 +/- 7 pmol/l (P less than 0.01), whereas both MCT and saline did not significantly increase plasma PP concentrations. Similarly, the integrated plasma PP secretion after LCT (1022 +/- 392 pmol/l per 90 min) was significantly greater than that after MCT (-690 +/- 358 pmol/l per 90 min; P less than 0.001) and that after saline (-462 +/- 205 pmol/l per 90 min; P less than 0.01). It is concluded that the secretion of PP in response to triglycerides is dependent on the chain length of the fatty acids.     

Evaluation of the postprandial effects of a fast-food meal on human plasma F(2)-isoprostane levels

Measurement of the F(2)-isoprostane, 8-epi-PGF(2alpha) is increasingly used as a sensitive and reliable marker of lipid peroxidation in vivo. Because the majority of 8-epi-PGF(2alpha) in plasma is associated with lipoproteins, it is possible that 8-epi-PGF(2alpha) derived from polyunsaturated fatty acid-rich food may become incorporated within these lipoproteins during synthesis and could contribute to the levels detected in plasma. In this study, we evaluated the postprandial effect of a single fast-food meal (McDonald's Big Mac meal, McDonald's Corp., London, England) on plasma total 8-epi-PGF(2alpha) in nine healthy subjects. Blood was collected before and 2 h postprandially. 8-Epi-PGF(2alpha) was measured by immunoaffinity extraction and gas chromatography-mass spectrometry. Fasting plasma 8-epi-PGF(2alpha) (875 +/- 25 pM) increased postprandially (956 +/- 23 pM, p <.05), although no significant change was observed in the normalized concentrations (2. 78 +/- 0.1 vs. 2.95 +/- 0.3 nmol/mmol arachidonic acid). Plasma lipid hydroperoxides, fatty acids, vitamin E, total antioxidant status, cholesterol, and triglycerides were not altered. Plasma glucose increased postmeal (4.4 +/- 0.1 vs. 4.9 +/- 0.1 mM, p <.05). These results indicate that the overall contribution of this lipid-rich meal to plasma 8-epi-PGF(2alpha) and other lipid peroxidation markers was small.     

Fatty acid metabolism and lipid secretion by perfused livers from rats fed laboratory stock and sucrose-rich diets

To assess the possible role of altered hepatic processing of free fatty acids in dietary sucrose-induced accumulation of triglyceride in the liver and blood plasma, livers from rats fed commercial laboratory stock and high sucrose diets were perfused both with and without oleic acid substrate. Consumption of the sucrose diet exerted a multiplicity of effects on oleic acid metabolism, characterized by decreased conversion to both ketone bodies and carbon dioxide, increased esterification into liver triglyceride, and increased secretion in triglyceride-rich lipoproteins. During the infusion of oleic acid, livers from sucrose-fed rats also exhibited decreased ketogenesis, and increased secretion of triglyceride from endogenous sources. Since oleic acid uptake from the perfusion medium was identical in both groups, the observed effects of sucrose feeding are ascribed to altered rates of intracellular metabolic processes. Mass and radiochemical analyses of perfusate ketone bodies and triglycerides were indicative of greater mobilization of triglycerides from hepatocellular lipid droplets in the livers from sucrose-fed rats. These livers contained more triglyceride and secreted more triglyceride even in the absence of infused oleic acid. In summary, the sucrose-rich diet increased the esterification:oxidation ratio of intracellular free fatty acids derived from both the circulation and endogenous sources within the hepatocyte. In response, secretion of triglyceride-rich lipoproteins by the liver and deposition of triglyceride within the liver were promoted. It is concluded that alterations in the processing of free fatty acids by the liver contribute significantly to the liver and plasma triglyceride accumulation following sucrose consumption.     

Distribution of radioactive glycerol and fatty acids among adipose tissue triglycerides after administration of glucose-U-14C

Adipose lipid obtained from fed rats 15 or 60 min after injection of radioactive glucose was separated into 10 triglyceride classes of differing fatty acid compositions. The distribution among these classes of total and radioactive triglyceride-glycerol was determined and found to be the same. Thus newly synthesized adipose triglycerides resemble in kind and proportion the triglycerides which exist in the tissue. This finding is in accord with the concept that the structures of adipose triglycerides are stable over long periods and that the turnover rate of the several triglyceride species are similar. After administration of radioactive glucose, the specific activity of saturated fatty acids was higher in the more saturated triglyceride species. These data indicate that newly formed saturated acids do not mix completely with all adipose tissue fatty acids available for esterification. Fatty acids derived from plasma triglyceride influenced the composition of newly synthesized adipose tissue triglyceride and thus constitute an important source of adipose tissue lipid.     

Myocardial triglyceride turnover and contribution to energy substrate utilization in isolated working rat hearts

The objective of this study was to determine the contribution of myocardial triglycerides to overall ATP production in isolated working rat hearts. Endogenous lipid pools were initially prelabeled (pulsed) by perfusing hearts for 60 min with Krebs-Henseleit buffer containing 1.2 mM [1-14C]palmitate. During a subsequent 60-min period (chase), hearts were perfused with either no fat, low fat (0.4 mM [9,10-3H] palmitate), or high fat (1.2 mM [9,10-3H]palmitate). All buffers contained 11 mM glucose. During the "chase," 14CO2 production (a measure of endogenous fatty acid oxidation) and 3H2O production (a measure of exogenous fatty acid oxidation) were determined. Oxidative rates of endogenous fatty acids during the chase were 279 +/- 50, 88 +/- 14, and 88 +/- 8 nmol of [14C]palmitate oxidized per g dry weight.min in the no fat, low fat, and high fat groups, respectively, compared to exogenous palmitate oxidation rates of 0, 361 +/- 68, and 633 +/- 60 nmol of [3H]palmitate/g dry weight.min, in the no fat, low fat, and high fat groups, respectively. Endogenous [14C]palmitate oxidation rates were matched by loss of [14C]palmitate from endogenous myocardial triglycerides. Overall triglyceride content decreased during the no fat and low fat chase perfusion but did not change during the high fat chase. Loss of triglyceride [14C]palmitate during the high fat chase was matched by incorporation of exogenous [3H]palmitate in triglycerides. In a second series of perfusions, three groups of hearts were perfused under similar conditions, except that unlabeled palmitate was used during the "pulse" and that 11 mM [2-3H/U-14C]glucose and unlabeled palmitate was present during the chase. During the chase, both glycolysis (3H2O production) and glucose oxidation (14CO2 production) rates were measured. Rates of glucose oxidation were inversely related to the fatty acid concentration in the perfusate (1257 +/- 158, 366 +/- 40, and 124 +/- 26 nmol of glucose oxidized per min.g dry weight in the no fat, low fat, and high fat groups, respectively), while rates of glycolysis were not significantly different between these groups. Calculation of overall ATP production from both oxidative and glycolytic sources determined that even in the presence of high concentrations of fatty acids, myocardial triglyceride turnover can provide over 11% of steady state ATP production in the aerobically perfused heart. In the absence of fatty acids, myocardial triglyceride fatty acids can become the major energy substrate of the heart.(ABSTRACT TRUNCATED AT 400 WORDS)     

Lipid metabolism during fasting

These studies were conducted to understand the relationship between measures of systemic free fatty acid (FFA) reesterification and regional FFA, glycerol, and triglyceride metabolism during fasting. Indirect calorimetry was used to measure fatty acid oxidation in six men after a 60-h fast. Systemic and regional (splanchnic, renal, and leg) FFA ([(3)H]palmitate) and glycerol ([(3)H]glycerol) kinetics, as well as splanchnic triglyceride release, were measured. The rate of systemic FFA reesterification was 366 +/- 93 micromol/min, which was greater (P < 0.05) than splanchnic triglyceride fatty acid output (64 +/- 6 micromol/min), a measure of VLDL triglyceride fatty acid export. The majority of glycerol uptake occurred in the splanchnic and renal beds, although some leg glycerol uptake was detected. Systemic FFA release was approximately double that usually present in overnight postabsorptive men, yet the regional FFA release rates were of the same proportions previously observed in overnight postabsorptive men. In conclusion, FFA reesterification at rest during fasting far exceeds splanchnic triglyceride fatty acid output. This indicates that nonhepatic sites of FFA reesterification are important, and that peripheral reesterification of FFA exceeds the rate of simultaneous intracellular triglyceride fatty acid oxidation.     

Quantitative aspects of free fatty acid metabolism in the fasted rat

Palmitate-1-(14)C was injected intravenously into unanesthetized, fasted rats. Disappearance of tracer from plasma free fatty acids was studied. A large component of free fatty acid (FFA) recycling was directly demonstrated by reinjection experiments. The latter studies also indicated the existence of an unidentified, rapidly turning over polar lipid in plasma which was synthesized from palmitate-(14)C. The appearance of (14)C in hepatic and extrahepatic triglycerides, in other esters, and in respired CO(2) was also followed. The data were analyzed using a multicompartmental model and a digital computer. Only a small fraction of the triglycerides formed in liver was derived directly from plasma free fatty acids. The major portion of net triglyceride formation appeared to be by way of an intermediate nontriglyceride ester pool which turned over relatively slowly compared to plasma free fatty acids. Initial approximations are as follows ( micromoles of fatty acid per min per 100 g body weight): net free fatty acid mobilization (irreversible disposal) = 2.4; hepatic triglyceride formation directly from plasma free fatty acid = 0.1; total hepatic lipid formation from plasma free fatty acids = 0.5; oxidation of free fatty acids to CO(2) = 0.8; percentage of respired CO(2) from direct oxidation of fatty acids = 12%; extrahepatic triglyceride formation directly from fatty acids = 0.4; total extrahepatic lipid formed directly from fatty acids = 1.2.     

Role of acetate in the reduction of plasma free fatty acids produced by ethanol in man

To investigate the mechanism by which ethanol lowers plasma free fatty acids, we tested the ability of two products of alcohol metabolism, acetate and lactate, to lower free fatty acids in man. Sodium acetate was given orally to five healthy fasting volunteers and caused a significant fall in plasma free fatty acids. After amounts of ethanol and acetate that produced similar reductions in free fatty acids, plasma acetate increased 3- to 4-fold within 20 min. In each of three subjects the fall of free fatty acids observed after acetate ingestion occurred at plasma acetate levels less than or equal to those reached after ethanol. In all studies plasma glucose remained stable. Oral administration of sodium lactate to another volunteer in amounts sufficient to raise plasma lactate concentrations to a level similar to that found after ethanol administration failed to lower plasma free fatty acids. Thus acetate, a metabolite of ethanol, reduces plasma free fatty acids at plasma acetate levels comparable to those resulting from ethanol metabolism, which suggests that the lowering of plasma free fatty acids produced by ethanol is mediated, at least in part, by acetate.     

Blocking VLDL secretion causes hepatic steatosis but does not affect peripheral lipid stores or insulin sensitivity in mice

The liver secretes triglyceride-rich VLDLs, and the triglycerides in these particles are taken up by peripheral tissues, mainly heart, skeletal muscle, and adipose tissue. Blocking hepatic VLDL secretion interferes with the delivery of liver-derived triglycerides to peripheral tissues and results in an accumulation of triglycerides in the liver. However, it is unclear how interfering with hepatic triglyceride secretion affects adiposity, muscle triglyceride stores, and insulin sensitivity. To explore these issues, we examined mice that cannot secrete VLDL [due to the absence of microsomal triglyceride transfer protein (Mttp) in the liver]. These mice exhibit markedly reduced levels of apolipoprotein B-100 in the plasma, along with reduced levels of triglycerides in the plasma. Despite the low plasma triglyceride levels, triglyceride levels in skeletal muscle were unaffected. Adiposity and adipose tissue triglyceride synthesis rates were also normal, and body weight curves were unaffected. Even though the blockade of VLDL secretion caused hepatic steatosis accompanied by increased ceramides and diacylglycerols in the liver, the mice exhibited normal glucose tolerance and were sensitive to insulin at the whole-body level, as judged by hyperinsulinemic euglycemic clamp studies. Normal hepatic glucose production and insulin signaling were also maintained in the fatty liver induced by Mttp deletion. Thus, blocking VLDL secretion causes hepatic steatosis without insulin resistance, and there is little effect on muscle triglyceride stores or adiposity.     

Cardiac function and metabolism in Type 2 diabetic mice after treatment with BM 17.0744, a novel PPAR-alpha activator

Hearts from diabetic db/db mice, a model of Type 2 diabetes, exhibit left ventricular failure and altered metabolism of exogenous substrates. Peroxisome proliferator-activated receptor-alpha (PPAR-alpha) ligands reduce plasma lipid and glucose concentrations and improve insulin sensitivity in db/db mice. Consequently, the effect of 4- to 5-wk treatment of db/db mice with a novel PPAR-alpha ligand (BM 17.0744; 25-38 mg x kg(-1) x day(-1)), commencing at 8 wk of age, on ex vivo cardiac function and metabolism was determined. Elevated plasma concentrations of glucose, fatty acids, and triacylglycerol (34.0 +/- 3.6, 2.0 +/- 0.4, and 0.9 +/- 0.1 mM, respectively) were reduced to normal after treatment with BM 17.0744 (10.8 +/- 0.6, 1.1 +/- 0.1, and 0.6 +/- 0.1 mM). Plasma insulin was also reduced significantly in treated compared with untreated db/db mice. Chronic treatment of db/db mice with the PPAR-alpha agonist resulted in a 50% reduction in rates of fatty acid oxidation, with a concomitant increase in glycolysis (1.7-fold) and glucose oxidation (2.3- fold). Correction of the diabetes-induced abnormalities in systemic and cardiac metabolism after BM 17.0744 treatment did not, however, improve left ventricular contractile function.     

Effect of enteral vs. parenteral glucose delivery on initial splanchnic glucose uptake in nondiabetic humans

To determine if enteral delivery of glucose influences splanchnic glucose metabolism, 10 subjects were studied when glucose was either infused into the duodenum at a rate of 22 micromol x kg(-1) x min(-1) and supplemental glucose given intravenously or when all glucose was infused intravenously while saline was infused intraduodenally. Hormone secretion was inhibited with somatostatin, and glucose (approximately 8.5 mmol/l) and insulin (approximately 450 pmol/l) were maintained at constant but elevated levels. Intravenously infused [6,6-(2)H(2)]glucose was used to trace the systemic appearance of intraduodenally infused [3-(3)H]glucose, whereas UDP-glucose flux (an index of hepatic glycogen synthesis) was measured using the acetaminophen glucuronide method. Despite differences in the route of glucose delivery, glucose production (3.5 +/- 1.0 vs. 3.3 +/- 1.0 micromol x kg(-1) x min(-1)) and glucose disappearance (78.9 +/- 5.7 vs. 85.0 +/- 7.2 micromol x kg(-1) x min(-1)) were comparable on intraduodenal and intravenous study days. Initial splanchnic glucose extraction (17.5 +/- 4.4 vs. 14.5 +/- 2.9%) and hepatic UDP-glucose flux (9.0 +/- 2.0 vs. 10.3 +/- 1.5 micromol x kg(-1) x min(-1)) also did not differ on the intraduodenal and intravenous study days. These data argue against the existence of an "enteric" factor that directly (i.e., independently of circulating hormone concentrations) enhances splanchnic glucose uptake or hepatic glycogen synthesis in nondiabetic humans.     

Fish oil decreases hepatic cholesteryl ester secretion but not apoB secretion in African green monkeys

Two groups of African green monkeys were fed diets containing 40% of calories as fat with half of the fat calories as either fish oil or lard. The fish oil-fed animals had lower cholesterol concentrations in blood plasma (33%) and low density lipoproteins (LDL) (34%) than did animals fed lard. Size and cholesteryl ester (CE) content of LDL, strong predictors of coronary artery atherosclerosis in monkeys, were significantly less for the fish oil-fed animals although the apoB and LDL particle concentrations in plasma were similar for both diet groups. We hypothesized that decreased hepatic CE secretion led to the smaller size and reduced CE content of LDL in the fish oil-fed animals. Hepatic CE secretion was studied using recirculating perfusion of monkey livers that were infused during perfusion with fatty acids (85% 18:1 and 15% n-3) at a rate of 0.1 mumol/min per g liver. The rate of cholesterol secretion was less (P = 0.055) for the livers of fish oil versus lard-fed animals (3.3 +/- 0.5 vs. 6.0 +/- 1.2 mg/h per 100 g, mean +/- SEM) but the rate of apoB secretion was similar for both groups (0.92 +/- 0.15 vs. 1.01 +/- 0.13 mg/h per 100 g, respectively). The hepatic triglyceride secretion rate was also less (P less than 0.05) for the fish oil-fed animals (8.3 +/- 2.5 vs. 18.3 +/- 4.4 mg/h per 100 g). Liver CE content was lower (P less than 0.006) in fish oil-fed animals (4.1 +/- 0.8 vs. 7.4 +/- 0.7 mg/g) and this was reflected in a lower (P less than 0.04) esterified to total cholesterol ratio of perfusate VLDL (0.21 +/- 0.045 vs. 0.41 +/- 0.06). The hepatic VLDL of animals fed fish oil had 40-50% lower ratios of triglyceride to protein and total cholesterol to protein. From these data we conclude that livers from monkeys fed fish oil secreted similar numbers of VLDL particles as those of lard-fed animals although the hepatic VLDL of fish oil-fed animals were smaller in size and relatively enriched in surface material and depleted of core constituents. Positive correlations between plasma LDL size and both hepatic CE content (r = 0.87) and hepatic VLDL cholesterol secretion rate (r = 0.84) were also found.(ABSTRACT TRUNCATED AT 400 WORDS)     

Quantification of triglyceride transport in blood plasma: a critical analysis.

Reliable and precise quantification of endogenous triglyceride transport in man has not been possible with simple means to date. Direct measurement of net splanchnic secretion of triglyceride fatty acids (TGFA) in very low density lipoproteins (VLDL) provides the must unambiguous information, but precision is low. Coupling infusion of labeled fatty acid with sampling of arterial and hepatic venous blood increases precision; however, the contribution of precursors other than plasma free fatty acids (FFA) must be assessed. Measurement of the rate of hydrolysis of plasma triglycerides after displacing lipases into the blood with heparin holds promise as a simple, nonisotopic method, but it has not been carefully validated and heparin itself alters FFA and triglyceride transport. Multicompartmental analysis following pulse injection of labeled fatty acid offers a practical approach, but uncertainties about the number and location of interacting compartments have made it impossible to determine an absolute value for transport. Reinjection of biologically labeled plasma VLDL is impractical for large scale use, and validity of this approach remains uncertain because of heterogeneity of VLDL-triglycerides and their complex metabolic behavior. Methods to label VLDL-triglycerides in vitro deserve more study as does labeling of other components, such as the B-apoprotein. Such approaches will require rigorous comparison with biologically labeled material as well as careful assessment of alterations in kinetic behavior that may occur when VLDL are separated from blood plasma.     

Differential effect of saturated and polyunsaturated fatty acids on hepatic glucose metabolism in humans

Prolonged infusions of lipid and heparin that achieve high physiological free fatty acid (FFA) concentrations inhibit hepatic (and peripheral) insulin sensitivity in humans. These infusions are composed largely of polyunsaturated fatty acids (PUFA; linoleic and linolenic). It is not known whether fatty acid composition per se affects hepatic glucose metabolism in humans. To address this issue, we examined the impact of enteral infusions of either palm oil (48% palmitic, 35% oleic, and 8% linoleic acids) or safflower oil (6% palmitic, 12% oleic, 74% linoleic acids) in 14 obese nondiabetic subjects. (2)H(2)O was administered to determine the contribution of gluconeogenesis to endogenous glucose production (EGP), and a primed continuous infusion of [6,6-(2)H]glucose was administered to assess glucose appearance. As a result of the lipid infusions, plasma FFA concentrations increased significantly in both the palm oil (507.5 +/- 47.4 to 939.3 +/- 61.3 micromol/l, P < 0.01) and safflower oil (588.2.0 +/- 43.0 to 857.8 +/- 68.7 micromol/l, P < 0.01) groups after 4 h. EGP was similar at baseline (12.4 +/- 1.8 vs. 11.2 +/- 1.0 micromol x kg FFM(-1) x min(-1)). During a somatostatin-insulin clamp, the glucose infusion rate was significantly lower (AUC glucose infusion rate 195.8 +/- 50.7 vs. 377.8 +/- 38.0 micromol/kg FFM, P < 0.01), and rates of EGP were significantly higher (10.7 +/- 1.4 vs. 6.5 +/- 1.5 micromol x kg FFM(-1) x min(-1), P < 0.01) after palm oil compared with safflower oil, respectively. Baseline rates of gluconeogenesis and glycogenolysis were also similar. However, after lipid infusion, rates of glycogenolysis were suppressed by safflower oil but not by palm oil. Thus these studies demonstrate, for the first time in humans, a differential effect of saturated fatty acids and PUFA on hepatic glucose metabolism.     

The incorporation of acetate,stearate and d(?)-β-hydroxybutyrate into milk fat by the isolated perfused mammary gland of the goat

1. Mammary glands of lactating goats were perfused for 12.5-15hr. with heparinized whole blood and infused with a substrate mixture of glucose, acetate and amino acids (and sometimes chylomicra) containing either [1-(14)C]acetate, d(-)-beta-hydroxy[1-(14)C]butyrate or [U-(14)C]stearate. 2. There was a substantial net uptake of acetate by the glands and transfer of radioactivity into milk fat. Acetate was extensively utilized for the synthesis of milk fatty acids of chain length up to C(14) and to a smaller extent for the synthesis of palmitate. 3. There was a small and variable net uptake of stearate and beta-hydroxybutyrate and negligible oxidation of these substrates. However, tissue uptake was demonstrated by a substantial fall in specific radioactivity across the glands and an extensive transfer of radioactivity into milk fatty acids. 4. With beta-hydroxybutyrate the labelling of milk fat was very similar to that with acetate, but the distribution of radioactivity suggested a cleavage into C(2) fragments of about 40%. 5. Labelled stearate gave rise to highly labelled stearate and oleate in the milk fat. Small amounts of radioactivity were detected in stearate of plasma triglycerides and oleate of plasma free fatty acids. 6. In experiments where there was a decline in milk-fat secretion late in perfusion, the milk fatty acids showed a marked decline in the proportion of stearate and oleate and a rise in the proportion of myristate and palmitate. This did not occur in experiments where milk-fat secretion was maintained at a higher level. 7. The present results confirm that there is a large pool of long-chain fatty acids in mammary tissue that can act as an endogenous source of these substrates.