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S B Clark

Publications and source records attributed to S B Clark.

At least 37 records · Page 2Linked to original sources
PubMed1991

A comparison of lipoprotein secretion, bile production and hepatic morphology in isolated rat livers perfused with a perfluorocarbon emulsion or rat erythrocytes.

Isolated rat livers were perfused with an oxygenated perfluorocarbon emulsion, FC-43 emulsion for 1 to 4 hr. FC-43 emulsion contained 20% FC-43 (wt/vol) perfluorotributylamine (the fluorocarbon component for the transport of oxygen and carbon dioxide) emulsified with 2.56% Pluronic F-68 (a nonionic surfactant) in Krebs-Ringer bicarbonate buffer. FC-43 emulsion also contained 3% hydroxyethyl starch as an oncotic agent and 1.8 mg/ml glucose. The viability (oxygen consumption), bile secretion, structural integrity and secretion of nascent lipoproteins by FC-43-perfused rat livers was compared with livers perfused with Krebs-Henseleit bicarbonate buffer that contained rat erythrocytes (25% hematocrit) and 1.5 mg/ml glucose (red blood cell medium). Oxygen consumption was somewhat higher in livers perfused with FC-43 emulsion. Bile secretion of livers perfused with FC-43 emulsion for 4 hr was reduced significantly to 40% of that by red blood cell medium. The structural integrity of livers perfused with FC-43 emulsion varied from normal to marked cellular damage. Light-microscopical examination of rat livers perfused with FC-43 emulsion showed ballooning of sinusoids, presence of vacuoles in sinusoidal lining cells in some hepatocytes and detachment of endothelium in sinusoids. The number of vacuoles progressively increased in longer perfusions. Electron-microscopical studies showed the presence of small (60 to 100 nm) vesicles of varying electron density, presumably fluorocarbon particles inside the vacuoles in sinusoidal lining cells (Kupffer and endothelial) and hepatocytes. After 4 hr of perfusion with FC-43 emulsion, most of the sinusoidal endothelia were denuded, and the microvilli of the hepatocytes all but disappeared. In contrast, the ultrastructure of rat livers perfused with red blood cell medium for 4 hr was unaltered. The accumulation of nascent lipoproteins in perfusates of FC-43-perfused livers was markedly reduced, and no normal very-low-density lipoprotein, low-density lipoprotein or high-density lipoprotein were isolated. Chemical analysis showed the presence of Pluronic F-68 in all lipoprotein fractions. Our data strongly suggest that, during recirculating liver perfusions with FC-43 emulsion (between 1 and 4 hr), the nonionic surfactant detergent Pluronic F-68 dissociated from the emulsion and markedly affected hepatic structure, lipoprotein secretion and the composition of lipoproteins isolated from perfusate. Therefore FC-43 emulsion is not a suitable liver-perfusion medium for studies of lipoprotein metabolism.

Animals↗
PubMed1988

The ionization and distribution behavior of oleic acid in chylomicrons and chylomicron-like emulsion particles and the influence of serum albumin.

A reproducible, fairly narrow-sized population of rat lymph chylomicrons, approximately 100 nm, was isolated by centrifugation and combined with low levels of [1-13C]oleic acid for NMR studies. The carboxyl chemical shift was monitored as a function of aqueous pH to characterize the ionization behavior of the fatty acid in these particles. The titration curves were very similar to those for oleic acid in equivalent-sized emulsion particles composed of egg phosphatidylcholine and triolein. A simple partition-ionization model was fitted to the data to derive values for apparent ionization constant, expressed as pKapp, of 7.4-7.5 and the "true" surface to core partition coefficient of approximately 7 for oleic acid in chylomicrons. The fatty acid in chylomicrons thus appeared to be largely associated with the surface regions of these particles. Addition of bovine serum albumin to the samples showed that near physiologic pH much of the fatty acid was bound to the albumin at fatty acid to albumin-binding stoichiometries as high as 5.1 and with mass ratios of greater than 2 in favor of the lipid or lipoprotein particles. Lowering the pH of the medium shifted the distribution of fatty acid away from albumin so that at pH 5 with the emulsion, virtually all the fatty acid was associated with the lipid. The behavior observed under physiologic conditions is consistent with the rapid clearance and redistribution of fatty acid generated in these particles by lipolytic processes. However, under conditions of severe acidosis, hyperlipidemia, and hypoalbuminemia a significant portion of fatty acids might be retained in triglyceride-rich lipoproteins and their remnants and affect subsequent metabolism.

Algorithms↗
PubMed1987

Phosphatidylcholine composition of emulsions influences triacylglycerol lipolysis and clearance from plasma.

Sonicated emulsions containing triolein, a specific phosphatidylcholine and cholesterol were prepared. Bolus doses were injected intravenously into rats and plasma clearance kinetics and organ uptakes were determined. Emulsion triacylglycerol lipolysis by rat heart lipoprotein lipase was measured in vitro. Phosphatidylcholine molecular species influenced emulsion metabolism in vivo and in vitro. Emulsions containing saturated phosphatidylcholines at temperatures below their melting points were poor substrates for lipoprotein lipase, compared with those stabilized by mixed chain phosphatidylcholines. Distearoylphosphatidylcholine stimulated hepatic uptake compared with emulsions made with egg yolk phosphatidylcholine, which modeled chylomicrons closely. Emulsion populations with the same surface compositions but with mean diameters of 700-800 A and 1100-1300 A were metabolized similarly, suggesting that, within the normal chylomicron size range, size alone does not determine the disposition of triacylglycerol-rich emulsions or lipoproteins.

Animals↗
PubMed1987

Plasma lipoproteins of free-ranging howling monkeys (Alouatta palliata).

1. Plasma lipids and lipoproteins of free-ranging howling monkeys from Costa Rica (Alouatta palliata), aged 5 months to 23 years, were characterized. 2. High density lipoproteins were lipid-rich, similar to HDL2 of human plasma. 3. Fatty acid compositions of major lipid classes of very low, low and high density lipoproteins differed among social groups, possibly due to both dietary and genetic factors. 4. Low and high density lipoprotein phospholipids were enriched in phosphatidylethanolamine. 5. Howler plasma cross reacted with antihuman apoA-I antibodies but not with antihuman LDL antibodies. 6. No dimeric form of apoA-II was present, unlike human apoA-II.

Aging↗
PubMed1987

Chylomicron metabolism by normal and injured isolated rat lungs.

Since the lung is the first highly vascularized organ to which chylomicrons are exposed, we sought to determine whether the lung vasculature is capable of metabolizing triglyceride contained in circulating, native chylomicrons. In addition, since acute lung injury can depress other endothelial cell associated metabolic functions, we determined whether acute injury due to alpha-naphthylthiourea (ANTU) changed chylomicron triglyceride metabolism by lungs. We compared the hydrolysis of radiolabelled chylomicrons from rat mesenteric lymph by perfused lungs isolated from rats pretreated with ANTU; with the vehicle, Tween 80, alone; or untreated control rats. In all groups of lungs, we found that perfusate content and concentration of triglyceride decreased over 30 minutes of perfusion, while that of free fatty acid increased, indicating that isolated lungs are able to hydrolyze chylomicron triglyceride. Despite enhancement of hydrolysis by perfusates containing 6 gm/100 ml of bovine serum albumin, there were no differences among the groups of lungs in the extent or rate of triglyceride metabolism. The [1-14C]-oleate from chylomicron triglyceride was taken up into lung tissue during 30 minutes of perfusion and incorporated into neutral lipid, phosphatidylcholine, and phosphatidylethanolamine to a similar degree by ANTU-injured and control lungs. Lipoprotein lipase activity in homogenates of lungs from ANTU and Tween treated rats did not differ. We conclude that lungs are capable of hydrolysis of triglyceride contained in chylomicrons and that this endothelial cell associated metabolic function is not altered by acute lung injury caused by ANTU.

Animals↗
PubMed1985

Influence of lecithin acyl chain composition on the kinetics of exchange between chylomicrons and high density lipoproteins.

The kinetics of lecithin exchange between native lipoproteins was characterized for individual molecular species of lecithins of rat mesenteric lymph chylomicrons and rat plasma HDL. Studies were performed in the absence of lipid transfer proteins. Donor (chylomicrons) and acceptor (HDL) particles were present in ratios of 1:1 and 1:10 with respect to total phospholipid. Biphasic exchange kinetics were observed for all major lecithins common to chylomicrons and HDL at both proportions of donor to acceptor particles. During the early rapid phase of exchange, complete in about 30 min, 40-60% of the total lecithin pool was exchanged. Initial exchange rates were most rapid for the more hydrophilic species of the major lecithins normally present in both lipoproteins. Calculated activation energies correspondingly were least for a diunsaturated lecithin (18:1-20:4), intermediate for lecithins were 16:0 in position-1 (16:0-18:2 and 16:0-20:4), and highest for analogous lecithins with 18:0 in position-1. A 10-fold increase in the ratio of acceptor to donor particles affected neither the biphasic nature of the exchange nor the rates of exchange of individual molecular species (consistent with exchange by diffusion rather than by particle collisions). Total equilibration of individual molecular lecithin species was achieved by 24 hr (37 degrees C, donor to acceptor ratio of 1:1) with only a small change in the relative mass of lecithins in chylomicrons and HDL. Novel lecithins containing 18:3, incorporated into chylomicrons, were found to exchange exceedingly rapidly.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Utilization of individual lecithins in intestinal lipoprotein formation in the rat.

To determine the molecular species composition of lecithins of different nascent lipoproteins, high density lipoproteins (HDL), very low density lipoproteins (VLDL), and chylomicrons (CM) were isolated from the mesenteric lymph of rats. Lymph was collected at 0 degrees C with 5,5'-dithiobis-2-dinitrobenzoic acid added to inhibit lecithin-cholesterol acyl transferase. CM were separated by ultracentrifugation and HDL from VLDL by dextran SO4-MG+2 precipitation. Molecular species of lecithin were directly isolated by reverse phase high performance liquid chromatography. In fasted animals, the lecithin compositions of lymph HDL and VLDL were virtually the same and closely resembled the lecithin composition of intestinal mucosa. When bile lecithin was eliminated (by bile diversion), there was a marked change in lecithin composition of all lipoprotein and mucosal samples, which was most notable for a reduction in 16:0-species (which are predominant in bile) and a relative increase in the corresponding 18:0-species. Feeding unsaturated triglycerides (triolein, trilinolein, or a combination of triolein and trilinolein) also resulted in a change in HDL and VLDL lecithin composition. The effect was similar whether bile lecithin was present or eliminated and was notable for a reduction in 16:0-species, an increase in 18:0-species, and the emergence of large amounts of diunsaturated lecithins that corresponded to the fatty acid composition of the triglycerides fed (i.e., 18:1-18:1, 18:2-18:2, and 18:1-18:2 lecithins). When bile-diverted rats were infused via the duodenum with a mix of [14C]choline-labeled lecithins (isolated from the bile of other rats), the incorporation of infused lecithins into different lymph lipoproteins was distinctly different. Individual lecithins were incorporated to a variable extent into each lipoprotein. In fasted rats the specific activities of all major molecular species of lecithin were relatively greater in VLDL than HDL, indicating that HDL derived proportionately more of its lecithins from an endogenous pool than did VLDL. Feeding triolein changed the specific activities of more of the lecithin species of VLDL than of HDL. The specific activities of lecithins in CM were more similar to VLDL than to HDL after triolein feeding. Results thus indicate that, although the lecithins of different mesenteric lymph lipoproteins are similar and may be derived from membrane sites with the same lecithin composition, lecithins incorporated into different lipoproteins originate from different metabolic pools and/or by different mechanisms.

Animals↗
PubMed1984

Reduced cholesterol transmucosal transport in rats with inhibited mucosal acyl CoA:cholesterol acyltransferase and normal pancreatic function.

Absorption of cholesterol during inhibition of mucosal acyl CoA:cholesterol acyltransferase was studied in mesenteric lymph fistula rats with normal pancreatic function. The specific inhibitor used (Sandoz Compound 58-035; 3-(decyldimethylsilyl)-N-[2-(4-methylphenyl)-1-phenylethyl]prop anamide) greatly reduced cholesterol esterification in vitro and decreased lymphatic secretion of esterified cholesterol in vivo, but did not affect triglyceride metabolism by the gut in vitro or in vivo. During steady state lymphatic transport of cholesterol, unesterified cholesterol was increased and cholesteryl esters were decreased in whole lymph, lymph chylomicrons, and very low density lipoproteins. Incorporation of labeled cholesterol infused into the lumen into cholesteryl esters of lymph very low density lipoproteins was particularly suppressed. Labeled cholesterol incorporation into individual cholesteryl esters differed and was differentially affected when total cholesteryl ester synthesis was inhibited. The results support a major regulatory role for mucosal acyl CoA:cholesterol acyltransferase in cholesterol absorption and imply differences in the metabolism of endogenous and exogenous cholesterol by the intestinal mucosa.

Acyltransferases↗
PubMed1984

Cholesterol absorption in rat intestine: role of cholesterol esterase and acyl coenzyme A:cholesterol acyltransferase.

Cholesterol esterase of pancreatic juice origin and acyl coenzyme A:cholesterol acyltransferase, both associated with the intestinal mucosa, are implicated in the extensive esterification of exogenous cholesterol during absorption. To assess the role of each enzyme, [4-14C]cholesterol absorption into mesenteric lymph of rats with normal mucosal levels of both esterification enzymes was compared with that of rats with normal acyl coenzyme A:cholesterol acyltransferase activity but deficient cholesterol esterase activity. The cholesterol esterase deficiency was accomplished by either surgical diversion of the pancreatic juice from the intestinal lumen or removal by specific immunoprecipitation of cholesterol esterase from the otherwise complete pancreatic juice. In the rats that were transferase-complete and esterase-deficient, cholesterol absorption into lymph and esterase activity in the mucosa were decreased an average of 83% and 75%, respectively, compared with rats complete with both esterification enzymes. Of the absorbed [4-14C]cholesterol in all rats, 82-90% was esterified and the mucosal levels of cholesterol esterase, even in the esterase-deficient rats, could readily account for this esterification. Because transferase activity was normal in rat intestine in which cholesterol esterase was deficient and cholesterol absorption was inhibited, transferase alone does not support the absorption of exogenous cholesterol in the absence of esterase. These results reconfirm the importance of esterification in the absorption of exogenous cholesterol and demonstrate that cholesterol esterase plays an essential role in the regulation of the absorption process.

Acyltransferases↗
PubMed1983

Dietary saturated fatty acid content affects lymph lipoproteins: studies in the rat.

We examined effects on intestinal absorption of cholesterol and triglycerides and intestinal lipoprotein formation by feeding rats diets in which saturated fatty acids (palmitic plus stearic) comprised 78%, 68%, 48%, or 38% of triglyceride fatty acids. Absorption into lymph of radiolabeled cholesterol was proportional to triglyceride absorption. The rates of absorption of these lipids were related inversely to the % saturated fatty acids fed. The distribution of newly absorbed cholesterol and triglyceride into intestinal lipoproteins differed. With increasing cholesterol absorption more was recovered in very low density lipoproteins in contrast to the appearance preferentially in chylomicrons of larger quantities of fatty acid. Lymph lipid content did not reflect a consistent pattern in relation to the experimental diet fed. The fatty acid composition of triglyceride-rich lymph lipoproteins resembled the diet closely. One-quarter of the intestinal lymph particles from rats fed the highly saturated diets was flattened and polygonal as judged by electron microscopy if cooled to room temperature; whereas with the same diets, particles collected and isolated at 37 degrees C were round. Proportions of A-I and C apolipoproteins in triglyceride-rich intestinal particles varied inversely; apoA-I increased as fat/cholesterol absorption was greater. Diet-induced alterations in plasma lipoproteins and increased circulating triglycerides in this study in rats were unrelated to the variations in intestinal absorption or lymph lipoprotein formation.

Animals↗
PubMed1982

Coordinate production of three exoenzymes of Staphylococcus staphylolyticus.

Staphylococcus staphylolyticus produced three exoenzymes (a staphylolytic endopeptidase, a hexosaminidase and a protease) coordinately under a range of conditions of induction and repression by various peptides and carbohydrates. Mutants of S. staphylolyticus were isolated and shown to have pleiotropic variations in the production of the three enzymes. Hypo- or hyperproducing mutants of one enzyme were invariably hypo- or hyperproducers for the other two enzymes. Mutants that had lost the ability to produce one of the exoenzymes invariably failed to produce the other two enzymes. Revertants isolated from non-producers that regained the ability to produce one of the exoenzymes always regained the ability to produce the other two as well. These results suggest that the three exoenzymes share a common regulatory or processing mechanism.

Endopeptidases↗
PubMed1982

Physical studies of d less than 1.006 g/ml lymph lipoproteins from rats fed palmitate-rich diets.

At body temperature the stable form of triglycerides rich in saturated fatty acids is crystalline. We examined the physical state of triglyceride-rich lymph lipoproteins from rats fed saturated fat, as a function of temperature. When chylomicrons and very low density lipoproteins were collected, isolated, and examined at 37 degrees C, they were liquid as judged by differential scanning calorimetry, x-ray diffraction analysis and proton nuclear magnetic resonance spectroscopy, and they appeared spherical by electron microscopy. At 23-26 degrees C, triglyceride began to crystallize in the alpha form, which transformed to the stable beta form at lower temperatures. On cooling from 23 degrees C to 17 degrees C, considerable crystallization occurred and the particle density was increased significantly. When lipoproteins were held at 0-7 degrees C, about 75% of the triglyceride crystallized, distorting the lipoprotein shape. Reheating from 0 degrees C to 37 degrees C left 25% of the triglyceride unmelted. Heating to 58 degrees C was necessary to melt all the crystallized triglyceride and to restore the spherical lipoprotein shape. After complete melting of cooled lipoproteins, the liquid state was maintained on recooling to 37 degrees C, with formation of a metastable particle similar to the nascent lipoprotein. Isolation of lipoproteins containing highly saturated triglyceride at temperatures below 23-26 degrees C results in partial crystallization, alters their physical properties, and may affect their metabolism.

Animals↗
PubMed1981

Effects of lecithin ingestion on plasma and lymph lipoproteins of normo- and hyperlipemic rats.

Rats were fed low- or high-cholesterol diets in which all fatty acids were derived from either polyunsaturated phosphatidylcholine (PC) or safflower oil (SO) for 15 days. In hypercholesterolemic rats, plasma high-density lipoprotein cholesterol esters were increased by dietary PC compared with SO. In normolipemic rats, PC administered either in the diet or by short-term, maximal-rate intraduodenal infusion had no effect on aortic or portal plasma lipoprotein lipids or on mesenteric lymph and plasma lipoproteins compared with SO. The phosphoryl component of PC, when absorbed by the enteral route, may influence lipoprotein profiles in hypercholesterolemic rats but has no effect in normolipemic rats.

Animals↗
PubMed1980

Pain responses in Nepalese porters.

When tested by the method of limits, Nepalese had much higher pain thresholds to electrical stimulation than Occidentals did. Discriminability was the same for both groups, however, indicating that there were no neurosensory differences. Nepalese had higher (stoical) criteria for reporting pain but were not less sensitive to noxious stimulation. The battery of sensory measurement procedures described may be applied to any modality and are particularly applicable to difficult field conditions.

Asian People↗
PubMed1980

Composition of lymph chylomicrons from proximal or distal rat small intestine.

The composition of lymph chylomicrons secreted by rat proximal and distal small intestine were compared during constant perfusion of a lecithin-stabilized tri (1-14C) oleoyl glycerol emulsion, in pairs of unanesthesized rats with mesenteric lymph fistulas. By the 6th hr of infusion when 14C-triglyceride output was constant, the distal intestine secreted 33% less chylomicron phospholipid. Distal chylomicrons were larger and had higher triglyceride:phospholipid and higher apoprotein:phospholipid ratios than chylomicrons secreted by proximal intestine. The major apoprotein classes--apoB, apoA-I, apoE(= ARP), and C peptides--were present in both groups of chylomicrons, but in different proportions. Distal chylomicrons contained less apoA-I and more C peptides, with an increase in apoC-III3, and a decrease in apoC-III0, compared with proximal chylomicrons. The present study suggests that the distal intestine is defective in the utilization of phospholipid from the intestinal lumen for chylomicron phospholipid synthesis. Whether the observed changes in the size and phospholipid or apoprotein content of distal chylomicrons affect their system metabolism is presently not known.

Animals↗
PubMed1979

Mucosal coenzyme A-dependent cholesterol esterification after intestinal perfusion of lipids in rats.

Coenzyme A-dependent esterification of cholesterol was determined in intestinal mucosal homogenates prepared after duodenal perfusion of cholesterol-free lipid emulsions for 5 h in unanesthetized rats. Cholesterol esterification rates were lowest and the mucosal cholesterol pool was greatly reduced after the same lipid infusions that, in lymph fistula rats, had produced chylomicrons deficient in cholesterol esters. CoA-dependent esterification rates were sufficient to account for all the cholesterol esters secreted in mesenteric lymph chylomicrons. During triglyceride secretion by the gut, unesterified cholesterol for chylomicron membranes may be maintained both by suppressing mucosal CoA-dependent cholesterol ester formation and from a mobilizable unesterified cholesterol pool within the mucosa.

Animals↗
PubMed1979

Apoprotein composition and turnover in rat intestinal lymph during steady-state triglyceride absorption.

Apoproteins of chylomicrons, very low density lipoprotein (VLDL), and a low density + high density fraction secreted by proximal and distal rat small intestine into mesenteric lymph were examined during triglyceride (TG) absorption. Apoprotein output and composition were determined and the turnover rates of labeled non-apoB (soluble) apoproteins in lipoprotein fractions were measured after an intraluminal [(3)H]leucine pulse during stable TG transport into lymph. The output of VLDL apoproteins exceeded that of chylomicrons during the absorption of 45 micro mol of TG per hour. More [(3)H]leucine was incorporated into VLDL than into chylomicrons and the decay of newly synthesized VLDL apoproteins was more rapid than that of chylomicrons, in part due to higher concentrations of apoA-I and apoA-IV with a rapid turnover rate. Chylomicrons from proximal intestine contained more apoA-I and less C peptides than chylomicrons from distal intestine. Ninety percent of [(3)H]leucine incorporated into soluble apoproteins was in apoA-I and apoA-IV, but little apoARP was labeled. The turnover rate of apoA-I and apoA-IV differed significantly in the lymph lipoproteins examined. Although total C peptide labeling was small, evidence for intestinal apoC-II formation and differing patterns of apoC-III subunit labeling was obtained. [(3)H]Leucine incorporation and apoprotein turnover rates in lipoprotein secreted by proximal and distal intestine were similar. The different turnover rates of apoA-I and apoA-IV in individual lipoproteins suggest that these A apoproteins are synthesized independently in the intestine.-Holt, P. R., A-L. Wu, and S. Bennett Clark. Apoprotein composition and turnover in rat intestinal lymph during steady-state triglyceride absorption.

Animals↗