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

Measuring very low density lipoprotein-triglyceride kinetics in man in vivo: how different the various methods really are

PURPOSE OF REVIEW

The purpose of this article is to briefly outline the methods that are currently available for the determination of very low density lipoprotein-triglyceride (VLDL-TG) kinetics in man in vivo.

RECENT FINDINGS

A number of novel methodologies have been developed over the years for quantifying VLDL-TG production, clearance, and turnover rates. Besides the splanchnic arteriovenous balance technique, tracer methods with radioactive and, more recently, stable isotopes have been widely used. Most of the latter approaches utilize an isotopically labelled substrate, such as glycerol, fatty acid or acetate, which is eventually incorporated into a VLDL-TG moiety, and monitor the time course of change in specific activity or enrichment. A procedure of in vivo labelling of VLDL-TG with stable isotopes and use of the labelled VLDL-TG as a tracer has also been described in man. There is, however, considerable variability in estimates of VLDL-TG kinetics obtained by the various techniques, which cannot be readily attributed to normal physiological variation. Still, a large part of this discrepancy may be related to differences in VLDL-TG pool size within the normal range, which seem to account for approximately 40-50% of the total variance in VLDL-TG kinetics in both men and women.

SUMMARY

Several methods are available for quantifying VLDL-TG kinetics in man in vivo, varying in the selection of tracer, mode of administration and sampling, and data analysis. These inherent features, along with different pool sizes, result in multifold variable estimations of VLDL-TG kinetic parameters.

Substitution of mixed amino acids resembling soy protein for mixed amino acids resembling casein in the diet reduces plasma cholesterol in slowly, but not rapidly fed nor fasted baboons

Studies of cholesterol and triglyceride metabolism were conducted under steady-state conditions in conscious restrained female baboons to ascertain whether the cholesterol- and triglyceride-lowering effects of soy protein v casein in nonrodents can be explained by differences in amino acid composition between the two proteins and whether the lipid-lowering effects of soy protein are influenced by fasting or the rate of feeding, ie, the nutritional state. The metabolic changes underlying the changes in serum cholesterol and triglyceride concentration were also investigated. Isocaloric diets consisting of 45% mixed amino acids (soy or casein type), 50% glucose and 5% corn oil, including all nutritional requirements, were infused intraduodenally for five days. There were no differences in lipid levels between the soy and casein diets in fasting baboons (days 4 or 5) or when the daily diet (calculated to equal 24 hours of energy utilization) was given rapidly by constant intraduodenal infusion over seven hours (day 4). During rapid feeding there was, unexpectedly, no significant change in plasma cholesterol concentration, but the estimated rate of cholesterol oxidation to bile acids fell significantly by 38 +/- 6% (from 39 +/- 4 to 24 +/- 3 mg/d). Significant differences between the soy and casein diets were observed only in the fed state produced by the slow constant isocaloric intraduodenal infusions of the diets [5.8 mg mixed amino acids (soy or casein type) plus 7.2 mg glucose/min/kg body wt0.75] on day 5. These diets were calculated to equal the simultaneous rate of energy utilization.(ABSTRACT TRUNCATED AT 250 WORDS)

Serial blood sampling from the portal and hepatic vein in conscious unrestrained miniature pigs

A technique of implantation of portal and hepatic venous catheters in miniature pigs is described in detail. To overcome early patency loss, Swan-Ganz balloon catheters were used. Portal as well as hepatic venous catheters could be held open to withdraw blood and to apply fluids over a period of at least 50 days. The collected hepatic venous blood represents mixed hepatic outflow. The method is recommended to study absorption of drugs and substrates from the gut and their metabolism in the liver of conscious unrestrained experimental animals.

Direct synthesis of low-density lipoprotein apoprotein B in the miniature pig

The metabolism of apoprotein B (apo B) was investigated in five miniature pigs following the injection of radiolabeled, very low-density lipoproteins (VLDL). The fractional catabolic rate (FCR) for VLDL apoprotein B was 0.71 +/- 0.10 h-1 (mean +/- SE), the rate of flux was 0.77 +/- 0.05 mg h-1 kg-1, and the pool size of apoprotein B averaged 1.26 +/- 0.20 mg kg-1. Examination of precursor-product relationships between VLDL and low-density lipoprotein (LDL) apoprotein B illustrated that a significant proportion (greater than 80%) of LDL apo B was derived from some source other than VLDL catabolism. In further experiments (n = 4), 125I-VLDL and 131I-LDL were simultaneously injected into miniature pigs. The fractional catabolic rate of LDL apo B averaged 0.055 +/- 0.008 h-1 and the flux rate 0.73 +/- 0.07 mg h-1 kg-1. These dual-label studies allowed us to calculate that an average of 16% of VLDL apoprotein B was converted to LDL and thus the remainder was cleared directly from the circulation. Simultaneous injection of radiolabeled homologous and human VLDL indicated that the catabolism of the two tracers was qualitatively similar. However, human VLDL apo B exhibited a slower fractional catabolic rate (0.42 v 0.71 h-1 P less than 0.05) and reduced rate of conversion to LDL. Therefore, low-density lipoproteins in the pig are largely produced by direct secretion into the circulation, independent of VLDL catabolism. Apo B metabolism in miniature pigs is similar to that of cynomologous and squirrel monkeys, and rats, but differs from normal humans in whom all LDL apo B is derived from VLDL catabolism.

Effects of dietary substitution of mixed amino acids for glucose on the splanchnic metabolism of plasma triglycerides, cholesterol, carbohydrates, and amino acids in conscious fed baboons

Splanchnic metabolism was studied in the fed state during prolonged constant intravenous administration of tracer amounts of [9,10]-3H palmitic acid and the calculated isocaloric intraduodenal administration (13 mg/min X kg body wt0.75) of either (1) glucose, (2) 15% mixed amino acids and 85% glucose or (3) 45% mixed amino acids and 55% glucose to conscious, restrained female baboons that had been maintained on a similar diet (supplemented in essential nutrients) for the previous 9 days. Secretion of plasma triglycerides from the splanchnic region was quantified from splanchnic flow and radiochemical measurements of transsplanchnic gradients of 3H-labeled free fatty acids and triglycerides. Mean splanchnic secretion of plasma triglycerides increased significantly as the proportion of dietary calories derived from amino acids was varied from 0 to 15 to 45% (mean values 1.1 +/- 0.1, 2.6 +/- 0.2 and 4.2 +/- 0.3 mumol/min kg body wt0.75, respectively, p less than 0.05). Increased triglyceride secretion was attributable to both significantly higher rates of esterification of free fatty acids taken up in the splanchnic region to triglycerides released into hepatic venous blood plasma (mean values 10 +/- 1, 16 +/- 2 and 34 +/- 5%, respectively) and to significantly higher rates of secretion of triglycerides derived from precursors other than free fatty acids. Higher intake of amino acids was also associated with both higher plasma concentrations of cholesterol and higher values for hepatic oxidation of cholesterol to bile acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Very low density lipoprotein metabolism in domestic pigs

Patterns of triacylglycerol (TG) turnover in plasma and liver, and the hepatic secretion of very density lipoprotein triacylglycerol (VLDL TG) into the circulation, have been studied in young Large White female pigs, using i.v. tracer [1,3-14C]- and [2-3H]glycerol. Serial measurements were made of plasma [14C]glycerol and [14C]glucose and of liver TG and plasma VLDL TG specific activities. In other studies VLDL TG obtained from a donor pig was reinjected into recipient animals to measure the early disappearance (dilution) of VLDL TG. Multicompartmental analysis revealed a mean rate of hepatic TG turnover somewhat slower than the rate of VLDL TG turnover, suggesting that almost all of the hepatic TG turnover was due to secretion of VLDL TG, and that the intestine probably contributed an appreciable part of the newly synthesized plasma VLDL TG. The t1/2 of reinjected VLDL TG, whether prepared by the most rapid possible techniques and reinjected immediately or stored for several days, was approximately 10 min. This was much faster than the t1/2 of the falling limb of the plasma VLDL TG curve seen after injection of labelled glycerol (t1/2 approximately 2 H). Thus, in these respects, the pig resembled all other species studied rather than human subjects as described by Farquhar et al.