Biomedical subjects

H G Wilcox

Publications and source records attributed to H G Wilcox.

At least 37 records · Page 2Linked to original sources

PubMed1992

In vivo growth hormone treatment stimulates secretion of very low density lipoprotein by the isolated perfused rat liver.

We have previously demonstrated in hepatocyte suspensions prepared after in vivo GH deprivation [hypophysectomy (hypox)] that rates of esterification of [1-14C]oleic acid into triglyceride (TG) and phospholipid (PL) were diminished, and that these esterification rates were correspondingly restored by repletion with recombinant GH. The current studies were designed to determine if GH exerts a similar effect on the secretion of very low density lipoprotein (VLDL), the primary plasma carrier of TG. We assessed rates of secretion of VLDL lipid and apoprotein by perfused livers prepared from cortisol/T3-replaced hypox female rats in the presence and absence of recombinant human (h) GH infusion. We also determined rates of synthesis and secretion of VLDL TG from infused [1-14C]oleic acid. After hypox, rates of secretion of VLDL lipid (TG, PL, and cholesterol) and apoprotein (total) were significantly decreased. In addition, VLDL secreted under these conditions was depleted of PL, relative to the other lipid components. Secretion of newly synthesized VLDL TG from [1-14C]oleic acid was also decreased; however, neither intracellular accumulation of labeled TG nor absolute tissue levels of TG were significantly changed. Conversely, GH treatment of hypox rats effectively restored rates of secretion of VLDL TG, PL, cholesterol (C) and apoprotein to control levels. These findings support the putative role of GH in regulating VLDL secretion in vivo by demonstrating that alterations in plasma GH are accompanied by changes in VLDL secretion. The findings further suggest that GH may regulate VLDL secretion by altering the amount of PL and/or apoprotein available for formation of the VLDL particle.

Animals↗

PubMed1992

Regulation of hepatic secretion of very low density lipoprotein by dietary cholesterol.

Male rats were fed a cholesterol-free diet or the same diet supplemented with either 0.05, 0.1, 0.25, 0.5, 1, or 2% C for 21 days to investigate the effects of cholesterol on secretion of very low density lipoprotein (VLDL). Cholesterol feeding increased plasma and hepatic concentrations of triglyceride (TG) and cholesteryl esters (CE) in a dose-dependent manner. Plasma VLDL and low density lipoprotein (LDL) lipids were elevated by cholesterol feeding, while the high density lipoprotein (HDL) lipids were reduced. The secretion of the VLDL by perfused livers from these cholesterol-fed rats was examined to establish the relationship between the accumulation of lipids in the liver and the concurrent hyperlipemia. Liver perfusions were carried out for 4 h with a medium containing bovine serum albumin (3% w/v), glucose (0.1% w/v), bovine erythrocytes (30% v/v), and a 10-mCi 3H2O initial pulse. Oleic acid was infused to maintain a concentration of 0.6 mM. Hepatic secretion of VLDL-TG, PL (phospholipid), free cholesterol (FC), and CE increased in proportion to dietary cholesterol and was maximal at 0.5% cholesterol in these experiments in which TG synthesis was stimulated by oleic acid. Secretion of VLDL protein and apoB by the perfused liver was also increased. The molar ratios of surface (sum of PL and cholesterol) to core (sum of TG and CE) lipid components of the secreted VLDL, regardless of cholesterol feeding, were the same, as were the mean diameters of the secreted particles. The molar ratios of surface to core lipid of VLDL isolated from the plasma also were not affected by cholesterol feeding. During perfusion with oleic acid of livers from the rats fed the higher levels of cholesterol, the hepatic concentration of CE decreased, while the level of TG was not changed. We conclude that the hypercholesterolemia and hypertriglyceridemia that occur in vivo from cholesterol feeding, concurrent with accumulation of CE and TG in the liver, must result, in part, from increased hepatic secretion of all VLDL lipids and apoB. The VLDL particles produced by the liver of the cholesterol-fed rat are assembled without modification of the surface lipid ratios (PL/FC), but contain a greater proportion of cholesteryl esters compared to triglyceride in the core, because of the stimulated transport of CE from the expanded pool in the liver.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

The role of selenium in the secretion of very-low-density lipoprotein in the isolated perfused rat liver.

A recirculating liver perfusion system was used to study the effects of dietary selenium (Se) on the hepatic secretion of very-low-density lipoprotein (VLDL). The perfusate from livers of rats fed on a Se-deficient diet incorporated about 50% more [1-14C]oleic acid into triacylglycerol (TG) and cholesteryl esters (ChoEs) than did the perfusate from livers of rats fed on a Se-supplemented diet. Similarly, livers from rats fed the Se-deficient diet secreted more VLDL and incorporated about 60% more [1-14C]oleic acid into VLDL TG and ChoEs than did livers from rats fed the Se-supplemented diet. The liver perfusate from rats in the Se-deficient group also showed significantly decreased fatty acid oxidation. We conclude that Se is a potent modulator of lipoprotein metabolism. A primary action of Se deficiency appears to be a decrease in fatty acid oxidation and a stimulation of fatty acid esterification, leading to increased VLDL TG and ChoEs formation and secretion.

Animals↗

PubMed1991

Effects of hyperthyroidism on synthesis, secretion and metabolism of the VLDL apoproteins by the perfused rat liver.

Livers from fed male Sprague-Dawley rats, made hyperthyroid by treatment with triiodothyronine (T3), were isolated and perfused in vitro. T3 (9.6 micrograms/day) was administered by osmotic minipump implanted intraperitoneally. Treatment with T3 for either 7 or 28 days reduced hepatic output of very-low-density lipoprotein (VLDL) and net synthesis of total associated apoproteins. After 7 days treatment, incorporation of [4,5-3H]leucine by livers from hyperthyroid rats into VLDL apo E was reduced while incorporation into apo B100, apo B48, and apo C's did not differ from euthyroid controls. The depressed incorporation of radioactivity into total VLDL protein was accounted for almost entirely on the basis of apo E. Incorporation of leucine into the total lipoprotein apo E isolated in the d less than 1.210 was also diminished by the hyperthyroid state, while that into apo B100, apo B48, and apo C in the total perfusate lipoprotein was similar to that of the euthyroid, as was found for the VLDL. Increased amounts of radioactive apo B100 and apo B48, however, were detected in the HDL fraction isolated from the medium perfusing livers from hyperthyroid rats. Hepatic uptake of VLDL protein and lipid was similar in euthyroid and hyperthyroid rats. Reduction of VLDL lipid and protein in the medium perfusing livers from T3-treated rats, therefore reflects hormonal action on synthesis and secretion, rather than uptake. Since the availability of apo B is thought to be required for secretion of VLDL, our observation suggests that synthesis of apo B is not depressed by treatment with T3 and that apoprotein synthesis is not a significant factor in the decreased output of VLDL by the liver, but that, as reported earlier, the lower output is a consequence of decreased synthesis of TG, the result of a diminished supply of hepatic glycero-3-phosphate in the hyperthyroid. The diminished amount of VLDL protein appears to be accounted for by the decreased quantity of apo E associated with a smaller VLDL particle secreted by livers from T3-treated rats.

Animals↗

PubMed1991

Metabolism of n-3 polyunsaturated fatty acids by the isolated perfused rat liver.

The hepatic metabolism of oleic acid and n-3 fatty acids (eicosapentaenoic acid, EPA and docosahexaenoic acid, DHA), and secretion of very low density lipoprotein (VLDL) were studied in isolated perfused rat livers from normal chow fed male rats. The basal perfusion medium contained 30% bovine erythrocytes, 6% bovine serum albumin (BSA), and 100 mg/dL glucose, in Krebs-Henseleit bicarbonate buffer (pH 7.4), which was recycled through the liver for 2 hr. Individual fatty acids (EPA, DHA or oleic acid), as complexes with 6% BSA, or albumin alone, were infused at a rate of 70 mumol/hr. When any of these fatty acids was infused at this rate, the ambient concentration in the medium was maintained at 0.3-0.4 mumol/mL, indicative of similar hepatic rates of uptake for each fatty acid (i.e., approximately 6 mumol/g liver/hr). When fatty acid was not infused, the ambient free fatty acid level was 0.16 mumol/mL. The concentrations of infused free fatty acids increased appropriately in the perfusion medium; however, with infusion of EPA, DHA, or oleate, the concentrations of perfusate palmitate and linoleate were the same as when fatty acid was not infused. Additionally, the perfusate concentration of oleate in the free fatty acid fraction was not affected by infusion of EPA and DHA. These data indicate a constant outflow of endogenous fatty acid unaffected by the presence of the exogenously supplied fatty acid. The net secretion rate of VLDL lipids and protein was stimulated by infusion of oleate, whereas when EPA was infused, secretion rates were lower and similar [except for VLDL cholesterol (C), which was greater] to those occurring when fatty acid was not provided.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Effects of thyroid status and fasting on hepatic metabolism of apolipoprotein A-I.

Metabolism of apolipoprotein (apo)A-I was studied in normal and chow-fed hyperthyroid rats, in 24-h fasted untreated male rats, and in rats after thyroparathyroidectomy (TXPTX). Rats were made hyperthyroid by administration of T3 (9.6 micrograms/day) or T4 (30 micrograms/day) with an Alzet osmotic minipump. Hyperthyroidism produced a similar two- to threefold elevation in plasma levels of apoA-I in male or female animals. During treatment with T3, plasma levels of T3 ranged from 200 to 400 ng/dl and did not correlate with plasma apoA-I levels. The net mass secretion and synthesis ([3H]leucine incorporation) of apoA-I by perfused livers from male hyperthyroid rats was elevated, while secretion of albumin was not different than that of euthyroid rats. Furthermore, the incorporation of [3H]leucine into total perfusate and hepatic protein was not altered by hyperthyroidism. The effect of thyroid hormone on apoA-I synthesis, therefore, does not appear to be a general effect on protein synthesis. After longer periods of treatment (28 days) with T3 (9.6 micrograms/day), hepatic apoA-I production decreased from that observed after 7 or 14 days of treatment, yet plasma apoA-I concentrations remained elevated. Plasma T3 decreased from 100 ng/dl to 40 ng/dl, in the hypothyroid rat resulting from TXPTX, but the plasma concentration of apoA-I did not change during the 2-week experimental period. The net secretion of apoA-I by livers from hypothyroid animals was depressed and albumin was uneffected compared to the euthyroid. Overnight fasting of euthyroid rats did not alter hepatic apoA-I secretion or plasma apoA-I levels, although under fasting conditions we had reported that hepatic output of apoB and E of VLDL is depressed. The addition of oleic acid to the perfusion medium, sufficient to stimulate VLDL production, did not affect net hepatic secretion of apoA-I by livers from euthyroid, hyperthyroid, or hypothyroid rats. In summary, hepatic synthesis of apoA-I appears to be controlled independently of other apo-lipoproteins and secretory proteins (albumin). Hepatic apoA-I synthesis is sensitive to thyroid status, increased in the hyperthyroid and decreased in the hypothyroid state. The specific stimulation of hepatic synthesis and secretion of apoA-I in the hyperthyroid state, however, tends to normalize over an extended period, perhaps from compensatory effects of a hormonal nature.

Animals↗

PubMed1990

Cholesterol is required for the secretion of the very-low-density lipoprotein: in vivo studies.

Rats were fed for 1 week with a standard chow diet, a diet supplemented with lovastatin (0.1%), or a diet supplemented with both lovastatin and cholesterol (0.1/0.1%), to study effects of depletion of a putative hepatic metabolic pool of cholesterol on secretion of the very-low-density lipoprotein (VLDL) in the intact animal. Triton WR-1339 (50 mg/100 g body wt.) or the 0.9% NaCl vehicle alone was given intravenously via a sacral vein. Treatment with lovastatin decreased the secretion of all plasma VLDL lipids, the average decrease after 2 h for VLDL triacylglycerol, phospholipid, cholesterol and cholesteryl ester being 45%. When both lovastatin and cholesterol were included in the diet, the secretion of VLDL triacylglycerol and phospholipid was similar to that of control animals, while secretion of VLDL cholesterol and cholesteryl ester was increased. Treatment with lovastatin reduced the hepatic concentration of cholesteryl esters 42% without affecting free cholesterol. In separate experiments, in vivo synthesis of cholesterol was determined 1 h after intraperitoneal administration of 3H2O. Incorporation into hepatic and plasma free cholesterol and cholesteryl esters was greater in the rats fed lovastatin than in control animals, concurrent with decreased VLDL secretion. The metabolism of VLDL was determined in vivo by intravenous administration of 125I-VLDL. The fractional clearance rates of 125I-VLDL from the plasma were similar among the three experimental groups. Synthesis of hepatic triacylglycerol from [1-14C]oleate in vivo was similar in all treatment groups; incorporation into plasma triacylglycerol was reduced with lovastatin treatment and reversed partially by inclusion of 0.1% cholesterol in the lovastatin diet. Plasma concentrations of triacylglycerol followed patterns of incorporation of [1-14C]oleate. Triacylglycerol concentration in the liver increased when cholesterol was included in the diet. In companion experiments, incorporation of [1-14C]oleate into perfusate triacylglycerol in vitro was reduced with perfused livers from lovastatin-treated animals. In these experiments, oxidation of fatty acid into CO2 and perchloric acid-soluble counts was not affected by lovastatin, added either to the diet or to the perfusate in vitro. It appears, therefore, that lovastatin does not affect triacylglycerol synthesis or fatty acid oxidation, which per se might reduce formation and secretion of VLDL. These data, therefore, strengthen the hypothesis that reduced availability of cholesterol in a putative hepatic metabolic pool, required for secretion and transport of triacylglycerol in the VLDL, is a factor contributing to decreased secretion of the VLDL.

Animals↗

PubMed1989

Cholesterol is required for secretion of very-low-density lipoprotein by rat liver.

To study potential effects of hepatic cholesterol concentration on secretion of very-low-density lipoprotein (VLDL) by the liver, male rats were fed on unsupplemented chow, chow with lovastatin (0.1%), or chow with lovastatin (0.1%) and cholesterol (0.1%) for 1 week. Livers were isolated from these animals and perfused in vitro, with a medium containing [2-14C]acetate, bovine serum albumin and glucose in Krebs-Henseleit buffer, and with an oleate-albumin complex. With lovastatin feeding, the hepatic concentrations of cholesteryl esters and triacylglycerols before perfusion were decreased, although free cholesterol was unchanged. However, hepatic secretion of all the VLDL lipids was decreased dramatically by treatment with lovastatin. Although total secretion of VLDL triacylglycerol, phospholipid, cholesterol and cholesteryl esters was decreased, the decrease in triacylglycerol was greater than that in free cholesterol or cholesteryl esters, resulting in secretion of a VLDL particle enriched in sterols relative to triacylglycerol. In separate studies, the uptake of VLDL by livers from control animals or animals treated with lovastatin was measured. Uptake of VLDL was estimated by disappearance of VLDL labelled with [1-14C]oleate in the triacylglycerol moiety, and was observed to be similar in both groups. During perfusion, triacylglycerol accumulated to a greater extent in livers from lovastatin-fed rats than in control animals. The depressed output of VLDL triacylglycerols and the increase in triacylglycerol in the livers from lovastatin-treated animals was indicative of a limitation in the rate of VLDL secretion. Addition of cholesterol (either free cholesterol or human low-density lipoprotein) to the medium perfusing livers from lovastatin-fed rats, or addition of cholesterol to the diet of lovastatin-fed rats, increased the hepatic concentration of cholesteryl esters and the output of VLDL lipids. The concentration of cholesteryl esters in the liver was correlated with the secretion of VLDL by the liver. These data suggest that cholesterol is an obligate component of the VLDL required for its secretion. It is additionally suggested that cholesteryl esters are in rapid equilibrium with a small pool of free cholesterol which comprises a putative metabolic pool available and necessary for the formation and secretion of the VLDL. Furthermore, the specific radioactivity (d.p.m./mumol) of the secreted VLDL free cholesterol was much greater than that of hepatic free cholesterol, suggesting that the putative hepatic metabolic pool is only a minor fraction of total hepatic free cholesterol.

Animals↗

PubMed1989

Stimulation of hepatic cholesterol biosynthesis by fatty acids. Effects of oleate on cytoplasmic acetoacetyl-CoA thiolase, acetoacetyl-CoA synthetase and hydroxymethylglutaryl-CoA synthase.

The effects of oleic acid on the activities of cytosolic HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) synthase, AcAc-CoA (acetoacetyl-CoA) thiolase and AcAc-CoA synthetase, as well as microsomal HMG-CoA reductase, all enzymes in the pathway of cholesterol biosynthesis, were studied in the isolated perfused rat liver. Oleic acid bound to bovine serum albumin, or albumin alone, was infused for 4 h at a rate sufficient to sustain an average concentration of 0.61 +/- 0.05 mM fatty acid during the perfusion. Hepatic cytosol and microsomal fractions were isolated at the termination of the perfusion. Oleic acid simultaneously increased the activities of the cytosolic cholesterol-biosynthetic enzymes 1.4-2.7-fold in livers from normal fed rats and from animals fasted for 24 h. These effects were accompanied by increased net secretion by the liver of cholesterol and triacylglycerol in the very-low-density lipoprotein (VLDL). We confirmed the observations reported previously from this laboratory of the stimulation by oleic acid of microsomal HMG-CoA reductase. In cytosols from perfused livers, the increase in AcAc-CoA thiolase activity was characterized by an increase in Vmax. without any change in the apparent Km of the enzyme for AcAc-CoA. In contrast, oleic acid decreased the Km of HMG-CoA synthase for Ac-CoA, without alteration of the Vmax. of the enzyme. The Vmax. of AcAc-CoA synthetase was increased by oleic acid, and there was a trend towards a small increase in the Km of the enzyme for acetoacetate. These data allow us to conclude that the enzymes that supply the HMG-CoA required for hepatic cholesterogenesis are stimulated, as is HMG-CoA reductase, by a physiological substrate, fatty acid, that increases rates of hepatic cholesterol synthesis and cholesterol secretion. Furthermore, we suggest that these effects of fatty acid on hepatic cholesterol metabolism result from stimulation of secretion of triacylglycerol in the VLDL by fatty acids, and the absolute requirement of cholesterol as an important structural surface component of the VLDL necessary for transport of triacylglycerol from the liver.

Acetate-CoA Ligase↗

PubMed1988

Stimulation of intestinal secretion of apolipoprotein AI by triiodothyronine.

Rats were made hyperthyroid with triiodothyronine in an osmotic minipump implanted intraperitoneally for one week. Intestinal lymph was collected from euthyroid and hyperthyroid rats for 40-55 hours. The rate of apolipoprotein AI (apo AI) secreted into the lymph by hyperthyroid rats exceeded that of euthyroid rats (149 +/- 9 vs 95 +/- 2 micrograms/hr), and was not associated with a proportional increase in the output of triglyceride. The fall in plasma concentration of apoAI in the euthyroid (10 mg/dl) and hyperthyroid (23 mg/dl) could be accounted for by the loss due to diversion of lymph. It appears that the intestine may contribute significantly to the increased plasma levels of apoAI observed in the hyperthyroid rats.

Animals↗

PubMed1988

Effects of oleic acid on the biosynthesis of lipoprotein apoproteins and distribution into the very-low-density lipoprotein by the isolated perfused rat liver.

The effects of oleic acid on the biosynthesis and secretion of VLDL (very-low-density-lipoprotein) apoproteins and lipids were investigated in isolated perfused rat liver. Protein synthesis was measured by the incorporation of L-[4,5-3H]leucine into the VLDL apoproteins (d less than 1.006) and into apolipoproteins of the whole perfusate (d less than 1.21). Oleate did not affect incorporation of [3H]leucine into total-perfusate or hepatic protein. The infusion of oleate, however, increased the mass and radioactivity of the VLDL apoprotein in proportion to the concentration of oleate infused. Uptake of oleate was similar with livers from fed or fasted animals. Fasting itself (24 h) decreased the net secretion and incorporation of [3H]leucine into total VLDL apoprotein and decreased the output of VLDL protein by the liver. A linear relationship existed between the output of VLDL triacylglycerol (mumol/h per g of liver) and secretion and/or synthesis of VLDL protein. Net output of VLDL cholesterol and phospholipid also increased linearly with VLDL-triacylglycerol output. Oleate stimulated incorporation of [3H]leucine into VLDL apo (apolipoprotein) E and apo C by livers from fed animals, and into VLDL apo Bh, B1, E and C by livers from fasted rats. The incorporation of [3H]leucine into individual apolipoproteins of the total perfusate lipoprotein (d less than 1.210 ultracentrifugal fraction) was not changed significantly by oleate during perfusion of livers from fed rats, suggesting that the synthesis de novo of each apolipoprotein was not stimulated by oleate. This is in contrast with that observed with livers from fasted rats, in which the synthesis of the total-perfusate lipoprotein (d less than 1.210 fraction) apo B, E and C was apparently stimulated by oleate. The observations with livers from fed rats suggest redistribution of radioactive apolipoproteins to the VLDL during or after the process of secretion, rather than an increase of apoprotein synthesis de novo. It appears, however, that the biosynthesis of apo B1, Bh, E and C was stimulated by oleic acid in livers from fasted rats. Since the incorporations of [3H]leucine into the VLDL and total-perfusate apolipoproteins were increased in fasted-rat liver when the fatty acid was infused, part of the apparent stimulated synthesis of the VLDL apoprotein may be in response to the increased formation and secretion of VLDL lipid.

Animals↗

PubMed1988

Stimulation of in vitro triglyceride synthesis in the rat hepatocyte by growth hormone treatment in vivo.

Hepatic fatty acid metabolism in the rat is sexually differentiated. Rates of esterification by the liver of fatty acid into triglyceride and other esterification products (phospholipid, diglyceride, cholesteryl esters) are higher in the female than in the male. There is evidence to suggest that GH feminizes other hepatic systems that exhibit sexual dimorphism, including hepatic steroid metabolism, PRL receptors, and estrogen binding. To investigate the role of GH in maintenance of the high rates of fatty acid esterification observed in the female, we assessed rates of [1-14C]oleic acid utilization by hepatocytes prepared from hypophysectomized (hypox) cortisol/T3-replaced female rats with an without continuous in vivo infusion of human (h) GH (5 micrograms/h). In addition, we assessed the effect of in vivo hGH treatment (5 micrograms/h) on [1-14C]oleic acid utilization in the normal male rat. Hypophysectomy was accompanied by a reduction in incorporation of [1-14C]oleic acid into products of esterification (triglyceride, phospholipid, diglyceride) and oxidation (CO2, ketone bodies). Continuous infusion of hGH (5 micrograms/h; 14 days) restored rates of fatty acid esterification in the hypox-cortisol/T3-replaced female rat, with the exception of cholesteryl esters. hGH infusion partially restored rates of fatty acid oxidation in the hypox cortisol/T3-replaced female rat. Treatment of the adult male rat with continuous infusion of hGH (5 micrograms/h; 7 days) resulted in increased rates of incorporation of [1-14C] oleic acid into triglyceride. In contrast, incorporation of oleic acid into phospholipid, diglyceride, and cholesteryl esters was unaltered. These results suggest that GH may be an important regulator of hepatic fatty acid metabolism.

Animals↗

PubMed1987

Composition and properties of very low density lipoproteins secreted by the perfused rat liver and subfractionated by affinity chromatography.

Very low density lipoproteins (VLDL) were isolated from the perfusate of rat livers infused with a complex of oleic acid bound to bovine serum albumin. Very low density lipoprotein (VLDL) secretion, bile flow, histopathology, and transmission electron microscopy indicated that secretory functions but not morphologic integrity of the livers were maintained during the procedure. Plasma VLDL and liver perfusate VLDL did not have similar size distribution. VLDL isolated from recycling perfusate and single pass perfusate were also subfractionated with concanavalin A-Sepharose 4B affinity chromatography. Three subfractions were eluted sequentially from the perfusate VLDL: a non-adherent fraction A and two adherent fractions B and C. The size of these VLDL, determined after negative staining and examination by transmission electron microscopy, was significantly decreased by affinity chromatography. VLDL in fractions A, B and C were spherical and had diameters of 935 +/- 17, 881 +/- 34 and 415 +/- 30 A respectively. Fraction A, which did not adhere to the column, contained 65% of the lipid applied to the column. The carbohydrate composition of fraction A VLDL was 11.2 +/- 0.6% fucose, 14.7 +/- 1.2% galactose, 43.7 +/- 2.3% N-acetylglucosamine, and 30.5 +/- 1.9% sialic acid. Sugars such as glucose and mannose, which bind to concanavalin A, were not detected. In contrast, VLDL fractions B and C, which adhered to the column, contained both glucose (17.7 and 2.5%) and mannose (5.8 and 8.3%) as well as the other sugars present in VLDL fraction A. Sodium dodecyl sulfate gradient gel electrophoresis revealed that the affinity column procedure clearly altered the apolipoprotein patterns of the applied VLDL, thereby producing abnormal fractions B and C. Fractions B and C also differed from unfractionated VLDL and fraction A VLDL in lipid composition, in surface/interior core lipid ratio, and in fatty acid composition of the interior core lipids, primarily triacylglycerols. The steady-state anisotropy, the limiting anisotropy and the lipid order parameter of fluorescence probe molecules 1,6-diphenyl-1,3,5-hexatriene and trans-parinaric acid incorporated into the VLDL were of the following order: fraction B greater than fraction A greater than fraction C. These results are consistent with the interpretation that concanavalin A-Sepharose 4B affinity chromatography may artificially produce a series of VLDL subfractions whose composition and structural properties do not resemble those of native VLDL.

Animals↗

PubMed1987

Secretion and uptake of nascent hepatic very low density lipoprotein by perfused livers from fed and fasted rats.

Livers from fed or 24-hr fasted male rats were perfused in a recycling system. VLDL labeled with [1-14C]oleate (95% in triglyceride), produced in separate perfusions of livers from fed rats, was added to the medium as a pulse. Uptake of VLDL 14C-labeled triglyceride by livers from fasted rats was less than that from fed rats regardless of addition of oleate. During the interval in which radioactive triglyceride was taken up, the mass of triglyceride in the medium increased, indicative of the synthesis and net secretion of triglycerides. The rates of secretion of VLDL and uptake of VLDL were both more rapid in livers from fed rats in comparison to those from fasted animals. It was calculated that about 50% of the triglyceride synthesized and secreted by the liver was taken back by livers from fed rats. The VLDL from livers of fasted rats did not contain any apoE detectable by SDS gel electrophoresis or by radioimmunoassay when no fatty acid or 166 mumol of oleic acid was infused. In contrast, apoE comprised 6% of the VLDL apoprotein derived from perfusion of livers from fed animals in the absence of added fatty acid, and 20% when the fed livers were infused with 166 mumol of oleic acid. However, the net output (accumulation) of apoE by fasted liver was only two-thirds that from fed livers. When lipoprotein-free rat plasma containing apoE (4 mg/dl) was used in place of bovine serum albumin, the VLDL secreted by livers from either fed or fasted rats contained apoE and was taken up to a similar extent by such livers. These data suggested that the apoE of the d greater than 1.21 g/ml fraction was transferred to newly secreted VLDL which then stimulated uptake of the VLDL by livers from fasted rats. With further stimulation of secretion of VLDL triglyceride by infusion of 332 mumol of oleic acid/hr, the percent of apoE in the VLDL secreted by livers from fasted rats increased to 20%, which was similar to that of the VLDL produced by livers from fed rats when either 166 or 332 mumol/hr was infused. These data suggest a relationship between rates of hepatic secretion of VLDL (TG) and apoE, and the association of apoE with the secreted VLDL. During fasting, reduced secretion of both VLDL and apoE resulted in a VLDL particle that was considerably diminished in content of apoE and, therefore, that would be taken up by the liver at a reduced rate, in comparison to that observed in the fed animal.

Animals↗

PubMed1986

HDL-induced cardiac prostacyclin synthesis: relative contribution of HDL apoprotein and lipid.

The objective of this study was to determine if the apoprotein or lipid constituents of high density lipoproteins (HDL) mediate HDL-induced prostacyclin synthesis in the Langendorff-perfused rabbit heart. Acetylation, acetoacetylation, or partial removal by trypsin digestion of HDL apoprotein did not reduce the ability of the lipoprotein to stimulate cardiac prostacyclin synthesis. Delipidated apoproteins were less effective in stimulating cardiac prostacyclin synthesis in comparison to intact HDL. In contrast, protein-free lipid vesicles, made from HDL lipids, caused a pronounced stimulation of cardiac prostacyclin synthesis. These results suggest that HDL apoproteins, in their native state, are not essential for HDL-induced cardiac prostacyclin synthesis. The stimulation of cardiac prostacyclin synthesis by HDL may depend on the lipoprotein's lipid rather than on its apoprotein constituents.

6-Ketoprostaglandin F1 alpha↗

PubMed1986

Effects of high-dose ethinyl estradiol on serum concentrations and hepatic secretion of the very-low-density lipoprotein, triacylglycerol, cholesterol, and apolipoprotein A-I in the rat.

Female and male rats were treated with ethinyl estradiol (5.0 mg/kg daily for 5 days). Control animals were pair fed to compensate for the reduction in food intake induced by the estrogen, or were fed ad libitum. Treatment with ethinyl estradiol reduced total cholesterol and apolipoprotein A-I concentrations in the serum of female and male animals. The concentrations of serum and hepatic triacylglycerol were depressed markedly in animals of both sexes in groups treated with ethinyl estradiol, compared to the control group fed ad libitum. Compared to the pair-fed controls, however, ethinyl estradiol had only a very minor further reduction on serum triacylglycerol concentration. In male and female rats, the synthesis and secretion of triacylglycerol by the liver was, in comparison to the pair-fed controls, stimulated by estrogen, whereas the secretion of unesterified cholesterol was unaffected by any of the treatment regimens. The synthesis and secretion of total cholesteryl esters by livers from male and female rats was increased by treatment with ethinyl estradiol. The hepatic synthesis and secretion of VLDL triacylglycerol and cholesteryl ester was stimulated by ethinyl estradiol in male and female rats, and the VLDL particle was enriched with cholesteryl ester. Treatment with the high-dose estrogen increased the secretion of apolipoprotein A-I by livers from female rats. It is suggested that the depression in the serum concentrations of cholesteryl esters and apolipoprotein A-I is the result of increased rates of hepatic and/or peripheral catabolism of these components and that the hepatic production rates were increased or unaffected in animals administered high doses of ethinyl estradiol. Since the secretion of apolipoprotein A-I by livers from male rats was unaffected by treatment with ethinyl estradiol, the response to estrogen may be sex related.

Animals↗

PubMed1986

High density lipoprotein-induced cardiac prostacyclin synthesis in vitro: relationship to cardiac arachidonate mobilization.

The objectives of this study were to characterize the effects of plasma lipoproteins on prostacyclin (PGI2) production by the Langendorff-perfused rabbit heart, and to determine the mechanism of lipoprotein-induced cardiac PGI2 production. PGI2 production by perfused rabbit hearts was stimulated by injections of rabbit very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL). HDL was much more effective than equivalent doses of VLDL or LDL. Infusion of HDL at a physiological concentration stimulated cardiac PGI2 output by 417%, but infusion of VLDL or LDL was ineffective. Cardiac PGI2 production increased from 47% to 340% with increasing doses of HDL. The release of cardiac PGI2 in response to injections or infusions of HDL occurred rapidly; maximal release of PGI2 was reached within 2 min after exposure to HDL. Injections of HDL stimulated the production of [3H]arachidonic acid, [3H]prostaglandin E2, [3H]prostaglandin F2 alpha, and [3H]6-keto-prostaglandin F1 alpha from hearts after prelabeling of cardiac lipids with [3H]arachidonic acid. These results indicate that plasma lipoproteins, specifically HDL, stimulate PGI2 production by the isolated rabbit heart. The mechanism by which HDL increases cardiac PGI2 production may involve the mobilization of cardiac arachidonic acid for PGI2 synthesis.

6-Ketoprostaglandin F1 alpha↗

PubMed1985

Stimulation by oleic acid of incorporation of L-[4,5-3H]leucine into very low density lipoprotein apoprotein by the isolated perfused rat liver.

Livers from normal fed or fasted (24h) rats were perfused in vitro to determine whether fatty acid affects the biosynthesis of very low density lipoprotein (VLDL) apoprotein. Oleate stimulated VLDL triacylglycerol output and increased incorporation of L-[4,5-3H]leucine into VLDL apoprotein in both the fed and fasted groups. The increased incorporation of [3H]leucine was mainly into VLDL-apoprotein E. The total mass of VLDL apoprotein secreted was also stimulated by oleate proportionately. These data suggest that fatty acids may stimulate hepatic synthesis and/or secretion of the VLDL apoproteins and that apo E, may be required for the formation and secretion of triacyl-glycerol in the VLDL.

Animals↗