Rat Low Density Lipoprotein Research Articles

Binding of homologous versus heterologous low density lipoproteins by different tissues of the rat

1. 1. The ability of five different tissues of the rat to bind homologous and heterologous (human) low density lipoprotein (LDL) was compared. 2. 2. Human LDL was as effective as rat LDL in displacing rat [ 125I]LDL from the plasma membrane fraction of rat skeletal muscle. 3. 3. Membranes from rat liver appeared to have no affinity for homologous LDL, while human LDL was slightly more effective in displacing labeled rat LDL from this tissue. 4. 4. A similar pattern of displacement was observed in membranes derived from rat heart. 5. 5. In contrast, membranes from both the aorta and adrenal glands of the rat displayed a significantly greater affinity for binding LDL of homologous origin. 6. 6. These results suggest that lipoprotein receptors analogous to those described for cultured human cells may exist in the rat and that individual tissues exhibit distinctly different affinities for binding LDL of both homologous and human origin.

Genes and nutrition in the regulation of human lipoprotein metabolism.

To illustrate the unitary and mutually inducive nature of clinical and basic research, five related studies of normal and abnormal lipoprotein metabolism in man are reported. 1. Regional lipoprotein metabolism was studied by 39 sets of arteriovenous difference measurements in the splanchnic and peripheral territories to localise and quantify some aspects of the production, conversion and catabolism of very low and low density lipoproteins. 2. By kinetic analyses in 43 patients with genetically defined hyperlipidaemias and in controls the metabolic bases of these disorders were partly characterized; the distinct kinetic mechanisms so defined and a genetic classification of primary hyperlipidaemia were shown to correspond, lending support to the validity of this classification. 3. In a patient with hypertriglyceridaemia due to failure to activate the enzyme lipoprotein lipase, resulting from the rare disorder of absence of apolipoprotein CII, the enzyme was activated acutely by infusion of a source of apo CII; serial changes in the concentration of each lipoprotein class were observed, confirming the protean effects of this enzyme on lipid transport. 4. The mechanisms by which commonly employed therapeutic diets influence hyperlipidaemia were studied by measuring lipoprotein synthesis and fractional catabolism in patients receiving high- and low-fat diets and different ratios of saturated and polyunsaturated fats; the kinetic basis in both instances was a change in synthetic rate of low density lipoprotein. 5. Lastly, studies are presented of the very wide individual differences in the degree of hypercholesterolaemia induced by cholesterol feeding. The efficiency of homeostasis against this challenge correlated with the degree to which the rate of cholesterol synthesis was down-regulated in isolated cells, and with the activity of high-affinity low density lipoprotein receptors on these cells. In subsequent twin studies this receptor activity was shown to be strongly influenced by genetic factors suggesting that this aspect of lipid homeostasis may largely be a familial characteristic.

Serum and hepatic nascent lipoproteins in normal and hypercholesterolemic rats.

The lipids and apoproteins of serum, hepatic Golgi cisternae, and secretory vesicle lipoproteins from hypothyroid, hypercholesterolemic rats were analyzed and compared to homologous lipoprotein fractions from euthyroid rats fed a low fat chow diet in order tao determine the nature of the nascent lipoprotein particles and indicate post-secretory modifications. Normal rat hepatic Golgi and secretory vesicles contained predominantly triglyceride-rich very low density lipoprotein (VLDL) which had little associated apoC-II or C-III and was deficient in apoE when compared to serum VLDL. Small quantities of cholesteryl ester-enriched low density lipoprotein (LDL) containing apoB and apoE were also present. Hepatic fractions from hypercholesterolemic rats contained cholesteryl ester- and apoE-rich, triglyceride-depleted VLDL of similar size, immunodiffusion characteristics, ratio of immunoassayable apoB to apoE, and lipid composition, to hypercholesterolemic serum VLDL. Hepatic levels of LDL in hypercholesterolemic rats were markedly elevated and enriched in cholesteryl ester and apoE when compare to normal hepatic LDL. Cholesteryl ester-rich hepatic VLDL and LDL increased in size and in cholesteryl ester and apoE content during transit from the Golgi cisternae into the secretory vesicles. Triglyceride-rich VLDL only acquired apoE which was further supplemented upon secretion. Nascent VLDL and LDL represented LpB-LpE association complexes and no deficiency in any apoE isoprotein within the cholesteryl ester-rich serum lipoproteins was observed. Thus, dietary-induced hypercholesterolemia in hypothyroid rats results in a fatty liver whose lipoprotein secretory products contribute to the plasma pool of abnormal cholesteryl ester- and apoE-enriched lipoproteins.

Saturable high affinity binding, uptake and degradation of rat plasma lipoproteins by isolated parenchymal and non-parenchymal cells from rat liver

1. Freshly isolated rat parenchymal and non-parenchymal liver cells bind iodinated rat very low density lipoprotein (VLDL) remnants, low density lipoproteins (LDL) and high density lipoproteins (HDL) in a saturable way. The apparent K m values for the binding of VLDL remnants are 6–20-fold lower than for LDL or HDL. The binding per mg cell protein to non-parenchymal cells is 5–8-fold higher than to parenchymal cells. Competition experiments indicate that rat VLDL-remnants, LDL and HDL, but not human LDL compete for the same surface receptor. It is concluded that the source of common recognition could be apolipoprotein E and that the interaction with the receptor is also influenced by the apolipoproteins A and C. The high apparent affinity of the receptor for VLDL remnants might be the result of multiple receptor occupancy of this lipoprotein. The presence of a 5–8-fold higher concentration of the described lipoprotein receptor in non-parenchymal cells as compared to parenchymal cells explains the relatively high uptake of VLDL remnants (as compared to LDL and HDL) as well as the relative contribution of parenchymal and non-parenchymal cells to the total hepatic uptake of lipoproteins in vivo. 2. The greater part (70–80%) of the parenchymal and non-parenchymal cell-associated apolipoproteins, LDL or HDL, remains bound to the external surface of the cells, during in vitro incubation at 37°C. High-affinity degradation of apolipoprotein(s) by isolated liver cells is dependent on the specific lipoprotein. During a 3 h incubation at 37°C, 37–49% of the total cell-associated 125I-abeled HDL is degraded. These percentages are 11–13% for 125I-labeled LDL and 4–8% for 125I-labeled VLDL remnants. Degradation of the different lipoproteins by non-parenchymal liver cells occurs at a 3–6-times higher rate per mg cell protein than by parenchymal cells. It is suggested that the rate-limiting step in the degradation of apolipoprotein by isolated liver cells is their transport to intracellular degradation sites.

The effect of thyroid hormones on low density lipoprotein receptors in cultured human skin fibroblasts (author's transl)

The catabolic rate of low density lipoprotein (LDL) has been shown to be delayed in hypothyroidism and increased in hyperthyroidism. LDL catabolism is mainly mediated through the LDL receptor pathway in the parenchymal cells. In the present paper the effect of thyroid hormones in LDL receptors was studied in cultured human skin fibroblasts. Human fibroblasts were established from 2 normal subjects, one heterozygous patient and 2 homozygous patients with familial hypercholesterolemia (FH). Human LDL (density 1.020 approximately 1.050 g/ml) from the serum of healthy subjects was prepared by differential ultracentrifugation. 125I-LDL was prepared by the ICl method of MacFarlane. Prior to incubation with 125I-LDL, cells were preincubated in a medium containing 5% lipoprotein-deficient serum obtained from a myxoedematous patient. Uptake and degradation of LDL by normal cells and FH heterozygous cells increased about 20% with 1.0 microgram/ml of L-triiodo-thyronine (T3) or L-thyroxine (T4). In FH homozygous cells, no increase of LDL receptor activity was observed with T3 or T4. These results suggest that thyroid hormones regulate LDL catabolism through the LDL pathway.

Metabolism of homologous and heterologous lipoproteins by cultured rat and human skin fibroblasts

Rat fibroblasts degraded human low density lipoprotein (LDL) very slowly, one-tenth to one-fortieth the rates observed in human fibroblasts. In rat cells, human LDL caused only very small increases in cell cholesterol content and acylCoA:cholesterol acyltransferase (ACAT) activity and caused only small decreases in beta-hydroxy-beta-methylglutaryl CoA (HMG CoA) reductase activity; in human cells, however, human LDL induced very large changes in all three of these parameters, as expected. The binding of human LDL to rat fibroblasts was not reduced by previous incubation with human LDL or with 25-hydroxycholesterol. Thus, in rat fibroblasts there appear to be few, if any, regulated high-affinity receptors that recognize human LDL. Rat LDL fractions (d 1.02-1.05 g/ml), in contrast, were degraded more rapidly than human LDL by rat fibroblasts, caused a significant increase in cell cholesterol content, an increase in ACAT activity, and a significant decrease in HMG CoA reductase activity. Moreover, the degradation of this rat LDL fraction by rat fibroblasts as a function of concentration was biphasic, i.e., there appeared to be a high-affinity component of degradation. Thus, it appears that rat fibroblasts do have a receptor for homologous lipoproteins. However, because both apoprotein B and apoprotein E are present in these rat lipoprotein fractions, the observed effects may relate to recognition of either or both of these apoproteins. The metabolism and metabolic effects of the conventionally defined high density lipoprotein (HDL) fraction of the rat by rat or human fibroblasts resembled those of human LDL in human fibroblasts. It is suggested that rat HDL may, because of its apo E content and higher concentration in rat plasma relative to that of LDL, play an important role in cholesterol homeostasis in vivo.

Time-related changes in the synthesis and secretion of very low density, low density and high density lipoproteins by cultured rat hepatocytes

Lipoproteins in the three major density classes were isolated from the medium of cultured rat hepatocytes incubated in the absence of serum for periods ranging from 1 to 48 h. De novo synthesis was suggested by the cyclo-heximide-sensitive incorporation of [ 3H]leucine into the apolipoproteins of the secreted lipoproteins. Hepatocyte d < 1.006 and d 1.006−1.063 g/ ml lipoproteins were similar to plasma very low density lipoprotein (VLDL) and low density lipoprotein (LDL), respectively, in chemical composition, morphology and apolipoprotein distribution. The isolation of plasma-like d 1.006−1.063 g/ ml particles is evidence for the hepatic origin of rat LDL; however, whether these particles are synthesized directly or result from catabolism of secreted VLDL has not been determined. Spherical d 1.063−1.21 g/ ml particles containing predominantly apolipoprotein A-I were isolated from the media. In contrast to plasma high density lipoprotein (HDL) the hepatocyte particles contained significant concentrations of triacylglycerol and apolipoproteins of M r > 100000 and lacked apolipoprotein A-IV. The pattern of lipoprotein secretion was related to the time of incubation. After incubation for 1, 3 and 6.5 h, VLDL comprised approx. 56% of the total lipoprotein mass, LDL 20% and HDL 24%. After 17 and 48 h the VLDL concentration was greatly reduced (approx. 20% of the total mass) while LDL and HDL concentrations were increased (33 and 47% of the total, respectively). Exogenous sodium oleate resulted in a concentration-dependent stimulation of VLDL synthesis at longer incubation periods. The triacylglycerol content of the secreted LDL fraction was also significantly increased following sodium oleate addition and there was an increased number of 425–650 Å particles present, which may represent catabolic products of VLDL. Hepatocyte mono-layers which can be maintained in serum-free media for extended periods should be useful for studying regulation of hepatic metabolism of the three major lipoprotein classes.

Disparities in the interaction of rat and human lipoproteins with cultured rat fibroblasts and smooth muscle cells. Requirements for homology for receptor binding activity.

This study characterizes the interactions of various rat and human lipoproteins with the lipoprotein cell surface receptors of rat and human cells. Iodinated rat very low density lipoproteins (VLDL), rat chylomicron remnants, rat low density lipoproteins (LDL), and rat high density lipoproteins containing predominantly apoprotein E (HDL1) bound to high affinity cell surface receptors of cultured rat fibroblasts and smooth muscle cells. Rat VLDL and chylomicron remnants were most avidly bound; the B-containing LDL and the E-containing HDL1 displayed lesser but similar binding. Rat HDL (d = 1.125 to 1.21) exhibited weak receptor binding; however, after recentrifugation to remove apoprotein E, they were devoid of binding activity. Competitive binding studies at 4 degrees C confirmed these results for normal lipoproteins and indicated that VLDL (B-VLDL), LDL, and HDLc (cholesterol-rich HDL1) isolated from hypercholesterolemic rats had increased affinity for the rat receptors compared with their normal counterparts, the most pronounced change being in the LDL. The cell surface receptor pathway in rat fibroblasts and smooth muscle cells resembled the system described for human fibroblasts as follows: 1) lipoproteins containing either the B or E apoproteins interacted with the receptors; 2) receptor binding activity was abolished by acetoacetylation or reductive methylation of a limited number of lysine residues of the lipoproteins; 3) receptor binding initiated the process of internalization and degradation of the apo-B- and apo-E-containing lipoproteins; 4) the lipoprotein cholesterol was re-esterified as determined by [14C]oleate incorporation into the cellular cholesteryl esters; and 5) receptor-mediated uptake (receptor number) was lipoprotein cholesterol. An important difference between rat and human fibroblasts was the inability of human LDL to interact with the cell surface receptors of rat fibroblasts. Rat lipoproteins did, however, react with human fibroblasts. Furthermore, the rat VLDL were the most avidly bound of the rat lipoproteins to rat fibroblasts. When the direct binding of 125I-VLDL was subjected to Scatchard analysis, the very high affinity of rat VLDL was apparent (Kd = 1 X 10(-11) M). Moreover, compared with data for rat LDL, the data suggested each VLDL particle bound to four to nine lipoprotein receptors. This multiple receptor binding could explain the enhanced binding affinity of the rat VLDL. The Scatchard plot of rat 125I-VLDL revealed a biphasic binding curve in rat and human fibroblast cells and in rat smooth muscle cells, suggesting two populations of rat VLDL. These results indicate that rat cells have a receptor pathway similar to, but not identical with, the LDL pathway of human cells. Since human LDL bind poorly to rat cell receptors on cultured rat fibroblasts and smooth muscle cells, metabolic studies using human lipoproteins in rats must be interpreted cautiously.

Catabolism of ( 125I)low density lipoproteins in isolated rat liver cells

Summary Isolated rat hepatocytes and non-parenchymal liver cells bind and internalize rat ( 125 I)low density lipoproteins via a binding mechanism with affinity for both low density lipoproteins and high density lipoproteins. The cells degrade ( 125 I)low density lipoproteins at a rate which is 30–75% higher than can be attributed to fluid endocytosis. Leupeptin, an inhibitor of thiol proteases including cathepsin B, reduced degradation of ( 125 I)low density lipoproteins with about 15–20% in both hepatocytes and non-parenchymal liver cells. Subcellular fractionation by means of sucrose gradient centrifugation of hepatocyte homogenates suggested that low density lipoproteins taken up in vivo accumulated in lysosomes. Low density lipoproteins taken up by hepatocytes in vitro were distributed in the gradient similarly to the plasma membrane marker enzyme 5′-nucleotidase.

Identification of specific high density lipoprotein-binding sites in rat testis and regulation of binding by human chorionic gonadotropin.

Freshly prepared rat testicular membranes bind iodinated rat high density lipoprotein (HDL) with high affinity (Kd = 32 microgram of HDL protein/ml). This high density lipoprotein binding differs from low density lipoprotein binding by cultured human fibroblast cells in two ways; it is not affected by Ca2+ or ethylenediaminetetraacetic acid, and it is not sensitive to pronase and trypsin. Testicular binding activity is primarily found in interstitial tissue containing Leydig cells and can be modulated by human chorionic gonadotropin administration in vivo (2-fold increase of binding capacity, with no affinity change) with 250 units/kg of human chorionic gonadotropin injection daily for 4 days. The interstitial high density lipoprotein binding site also recognizes rat very low density lipoprotein, but not rat low density lipoprotein, as shown by displacement experiments. When membrane preparations of various other tissues were assessed for their high density lipoprotein binding, we found that the adrenal density lipoprotein binding, we found that the adrenal gland binds rat high density lipoprotein with similar affinity and capacity, while spleen, kidney, and heart showed no high affinity binding. In addition, when iodinated rat low density lipoprotein was tested for its ability to bind to testicular membranes, no high affinity saturable binding was observed. We conclude that there are specific high density lipoprotein-binding sites present in steroidogenic tissues; these binding sites are not found in the nonsteroidogenic tissues tested. Furthermore, no high affinity low density lipoprotein-binding sites can be demonstrated in the testis; thus it appears that high density lipoprotein, rather than low density lipoprotein, is the major cholesterol-carrying lipoprotein recognized by the rat testis.

Uptake and degradation of rat and human very low density (remnant) apolipoprotein by parenchymal and non-parenchymal rat liver cells

1. 1. The relative contribution of parenchymal and non-parenchymal cells to the in vivo hepatic uptake of serum apolipoproteins was measured 30 min after intravenous injection of radioiodinated rat very low density lipoprotein (VLDL) remnants, rat and human VLDL, low density lipoprotein (LDL) and high density lipoprotein (HDL). Using rat VLDL, VLDL-remnants, LDL and HDL, respectively, the non-parenchymal cells contain 4.7, 4.9, 6.1 and 5.3 times the amount of trichloroacetic acid-precipitable radioactivity per mg cell protein as compared to parenchymal cells. For human VLDL, LDL and HDL these values are 5.1, 12.0 and 5.9, respectively. 2. 2. The abilities of homogenates of human liver, rat liver parenchymal cells and rat liver non-parenchymal cells to hydrolyze human and rat iodinated VLDL apoprotein were determined by measuring the amount of trichloroacetic acid-soluble (non-iodide) radioactivity liberated upon incubation at the optimal pH of 4.2. Non-parenchymal cells possess a 8–21-fold higher maximal capacity to degrade VLDL apoprotein per mg of cell protein than parenchymal cells. This factor is 5–6 for VLDL-remnant apoprotein degradation measured at low (suboptimal) apolipoprotein concentrations. 3. 3. It is concluded that, in addition to parenchymal cells, the non-parenchymal cells play an important role in the hepatic uptake and possibly degradation of VLDL-(remnant) apoprotein.

Inhibition of receptor-mediated clearance of lysine and arginine-modified lipoproteins from the plasma of rats and monkeys.

Reductive methylation of at least 30% of the lysine residues or 1,2-cyclohexanedione modification of 45% of the arginine residues prevented low density lipoproteins (LDL) from binding to cell surface receptors of fibroblasts in vitro, without significantly altering other physical or chemical properties of the LDL. When rat or human LDL with more than 30% of the lysine residues methylated were injected intravenously into rats, the clearance of these lipoproteins from the plasma was slowed considerably. The half-life of the reductively methylated LDL was approximately twice that obtained for control (unmodified) LDL, and the value for the fractional catabolic rate was approximately half that of the control. Furthermore, when human LDL modified by reductive methylation were injected into rhesus monkeys, the rate of clearance was similarly retarded, and the value for the fractional catabolic rate was reduced by approximately 50% as compared with the value for control LDL. A dual isotope labeling technique ((125)I and (131)I) was used to compare the disappearance of the control and modified LDL in the same animal. It was demonstrated that not only modification of lysine residues but also modification of the arginine residues with 1,2-cyclohexanedione retarded the plasma clearance of the rat LDL. However, the cyclohexanedione modification was spontaneously reversible at 37 degrees C, whereas reductive methylation of the lysine residues was stable. It is concluded that the selective chemical modification of lysine or arginine residues of LDL interferes with the normal uptake of these lipoproteins in vivo as well as by fibroblasts in vitro. These data provide an estimation of the level of receptor-mediated clearance of LDL from the plasma, a value that may be as high as 50% in rats and monkeys.

The hydrolysis of cholesterol esters in plasma lipoproteins by hormone-sensitive cholesterol esterase from adipose tissue.

Adipose tissue contains a high level of neutral esterase active against emulsions of cholesteryl oleate. The present studies show that this enzyme can also effectively hydrolyze the cholesterol esters in native rat plasma high density lipoproteins (HDL) and low density lipoproteins (LDL). The hydrolysis of lipoprotein cholesterol esters by a pH 5.2 isoelectric precipitate fraction from the freshly prepared 100,000 X g supernatant of chicken adipose tissue was low, but increased more than 50-fold on activation with cyclic AMP-dependent protein kinase. Rat adipose tissue homogenates were also very active against lipoprotein cholesterol esters, hydrolyzing as much as 60% of the total labeled cholesterol ester in HDL or LDL in 1 h. Activity was optimal at pH 7 and very low at pH 4. No protease activity was detected at pH 7 and, since assays were done in 2 mM EDTA, phospholipase A activity was presumably negligible. The results show that hormone-sensitive cholesterol esterase of adipose tissue has ready access to the neutral lipid core of plasma lipoproteins, either because the enzyme penetrates the polar shell or because the cholesterol ester in the core is exposed, at least intermittently, to allow enzyme-substrate complex formation. Whether or not this enzyme activity plays a role in lipoprotein degradation by adipose tissue remains to be determined.

In vivo uptake of human and rat low density and high density lipoprotein by parenchymal and nonparenchymal cells from rat liver

The relative contribution of the parenchymal and nonparenchymal rat liver cells to the hepatic uptake of human and rat high density lipoprotein (HDL) and low density lipoprotein (LDL) was determined in vivo. Non-parenchymal cells, isolated 6 h after intravenous injection of iodinated human HDL and LDL, contained respectively 4.2 and 6.3 times the amount of trichloroacetic acid-precipitable radioactivity per mg cell protein as compared to parenchymal cells. For rat iodinated HDL and LDL these factors were 3.4 and 4.1, respectively. These results indicate that nonparenchymal liver cells play a substantial role in the hepatic uptake of human and rat HDL and LDL in vivo.

Catabolism of the apoprotein of low density lipoproteins by the isolated perfused rat liver.

Isolated rat livers were perfused for 4 hours in a recirculating system containing washed rat erythrocytes. Biologically screened, radioiodinated low density lipoproteins (1.030 < d < 1.055 g/ml) were added to the perfusate with different amounts of whole serum to supply unlabeled rat low density lipoproteins. Apolipoprotein B contained 90% of the bound (131)I, other apolipoproteins contained 4%, and lipids contained the remainder. The fraction of apolipoprotein mass degraded during the perfusion was quantified by the linear increment of non-protein-bound radioiodine in the perfusate, corrected for the increment observed during recirculation of the perfusate in the absence of a liver. The fractional catabolic rate ranged from 0.3 to 1.7%/hr in seven experiments and was inversely related to the size of perfusate pool of low density apolipoprotein. The catabolic rate of low density apolipoprotein (fractional catabolic rate x pool size) in four livers, in which the concentration of rat low density lipoproteins was 50-100% of that present in intact rats, was 5.3 +/- 2.7 micro g hr(-1) (mean +/- SD). Similar results were obtained with human low density lipoproteins. These rates were compared with catabolic rates for the apoprotein of rat low density lipoproteins in intact animals. Fractional catabolic rate in vivo, obtained by multi-compartmental analysis of the disappearance curve of (131)I-labeled low density apolipoprotein from blood plasma, was 15.2 +/- 3.1% hr(-1) (mean +/- SD). Total catabolic rate in vivo (fractional catabolic rate x intravascular pool of low density apolipoprotein) was 76 +/- 14 micro g hr(-1) (mean +/- SD). The results suggest that only a small fraction of low density apolipoprotein mass in rats is degraded by the liver.

Iodine labeled human and rat low-density and high-density lipoprotein degradation by human liver and parenchymal and non-parenchymal cells from rat liver

1. 1. The abilities of homogenates of human liver, rat liver parenchymal cells, rat liver non-parenchymal cells and total rat liver to catabolize human and rat iodinated high-density lipoprotein (HDL) and low-density lipoprotein (LDL) were determined by measuring the amount of trichloroacetic acid-soluble (noniodide) radioactivity liberated upon incubation at the optimum pH of 4.2. 2. 2. A comparison of the capacities of human liver, rat liver and parenchymal and non-parenchymal cells from rat liver indicated that these different preparations are all able to degrade rat iodine-labeled LDL and HDL, with a 5–6-fold higher capacity for HDL as compared to LDL. 3. 3. Iodine-labeled human HDL can be degraded by rat liver, rat parenchymal and rat non-parenchymal cells with a 5–7-fold higher rate than human iodinelabeled LDL. Human liver homogenate was more active in the degradation of both human and rat iodine-labeled LDL and rat HDL as compared to rat liver. 4. 4. A comparison of the capacities of parenchymal and non-parenchymal cells for the degradation of iodine-labeled human and rat LDL and HDL indicates that non-parenchymal cells possess a considerable higher capacity to degrade these lipoproteins per mg of cell protein. 5. 5. The results indicate that a high proportion of the total rat liver capacity for lipoprotein degradation is localized in the non-parenchymal liver cells and this, together with the active endocytic activity, suggests an important role of these liver cells in hepatic lipoprotein catabolism.

Discrepancies in the catabolic pathways of rat and human low density lipoproteins as revealed by partial hepatectomy in the rat

The serum decay of iodine-labeled low density lipoprotein (LDL) was measured in rats 2–8 h after partial hepatectomy or sham-operation. In sham-operated rats homologous LDL showed a biexponential decay with t 1 2 ' s of 0.27 ± 0.02 h and 4.3 ± 0.8 h for the initial rapid phase and the slow phase, respectively. After partial hepatectomy the t1/2 of the rapid phase was unchanged, while the t 1 2 of the slow phase increased to 10.3 ± 1.0 h ( P < 0.01). The fractional catabolic rate (F.C.R.) was decreased by 62% (from 4.8 ± 0.9 to 1.8 ± 0.2/day), a % comparable to the amount of liver removed. The level of serum LDL is slightly elevated (19%), if measured 8 h after partial hepatectomy. Chemical analysis shows that the triglyceride content of LDL decreases from 9.4 ± 2.3% in control rats to 3.7 ± 0.9% after partial hepatectomy. Human iodine-labeled LDL shows a monoexponential decay from rat serum with a t sol1 2 of 9.4 ± 0.7 h, which is not affected by partial hepatectomy. The F.C.R. is low in both sham-operated and hepatectomized rats (1.8 ± 0.1 vs. 1.9 ± 0.1/day). The % of the injected rat LDL radioactivity recovered in the liver of control rats is twice as high as that of human LDL, as measured 6 h after the injection; at this time the serum radioactivity is much lower in the case of rat LDL. Partial hepatectomy did not change the levels of extrahepatic tissue radio-activities significantly. It is concluded that the liver determines the catabolic rate of homologous LDL-apoproteins in the rat. In contrast, the slow degradation of human LDL-apoprotein is probably located extrahepatically.

Synthesis of serum very low density lipoprotein in rats fed a low whole egg protein diet.

Synthesis of serum very low density lipoprotein in rats fed a low whole egg protein diet.

Effect of a dietary rapeseed protein concentrate on the contents of alpha-tocopherol and zinc in serum, liver and tibia of rats.

In order to study the effects of a dietary rapeseed protein concentrate (RPC) on tocopherol and zinc levels in serum, liver and tibia, female rats were fed a tocopherol-reduced diet for a 3-week period and were then transferred to either of a RPC diet or a control diet containing casein at 20% levels of protein. No unfavourable effects on food intakes or weight gains were recorded. The serum zinc content of the RPC group was slightly reduced and the serum tocopherol values significantly lowered in comparison to the control group. Zinc and tocopherol contents of the livers were not changed. Precipitation of sera by dextran sulphate in the presence of calcium ions indicated a reduced level of low density lipoproteins in rats fed RPC diet. In a second experiment, male and female rats were fed diets containing 10% of protein derived either from RPC or casein. Low zinc contents of tibiae indicated a low biological availability of the zinc present in the RPC diet. Rats on RPC diet also showed reduced levels of tocopherol in serum. Despite a lowered zinc status in rats fed RPC diet there was no reduction in food intake, weight gain or growth of tibia in these groups.

Increase of serum very low density lipoproteins in rats after administration of alpha-hexachlorocyclohexane.

After enteral administration of 200 mg/kg alpha-hexachlorocyclohexane (alpha-HCH) female Wistar rats develop a hyperlipemia. 48 h after administration of alpha-HCH, serum triglycerides are increased by 300%, whereas both serum cholesterol and serum total phospholipids only increase by about 45%. Serum free fatty acids are not significantly altered. Fractionation of the serum lipoproteins by ultracentrifugation shows that the hyperlipemia is due to a fivefold increase in serum very low density lipoproteins. Hepatic triglyceride secretion, calculated after i.v. injection of Triton WR 1339, is increased in animals pretreated wtih alpha-HCH. Corresponding to this observation, drugs known to diminish the triglyceride secretion of the liver, such as actinomycin D, cycloheximide; glucagon, orotic acid, CFT 1201, and CFT 1042 reduce the alpha-HCH-induced hyperlipemia. We concluded from the results that hyperlipoproteinemia after alpha-HCH is due to an increased hepatic very low density lipoprotein secretion. At the same time, the blood sugar level was decreased in fasting animals after treatment with alpha-HCH. Earlier experiments suggest that this effect is due to a decreased gluconeogenesis in the liver.