Biomedical subjects

N O Davidson

Publications and source records attributed to N O Davidson.

At least 91 records · Page 5Linked to original sources

PubMed1990

Apolipoprotein B mRNA editing is modulated by thyroid hormone analogs but not growth hormone administration in the rat.

Recent work has demonstrated that the unique post-transcriptional editing reaction which modifies mammalian apolipoprotein (apo) B100 mRNA, producing an in-frame stop codon in the modified (apo B48) transcript, is modulated in vivo in the rat liver by thyroid hormone (T3). We now report the results of studies undertaken to examine the effects of two synthetic T3 analogs and GH on apo B gene expression together with their effects on hepatic apo A-I, A-IV, C-III, and malic enzyme (ME)mRNAs. The T3 analogs were previously shown to exhibit similar binding to the hepatic nuclear T3 receptor (50% and 38% of native T3) but differing biopotency (18 and less than 3% of native T3). Apo B100 mRNA editing, determined by differential hybridization of polymerase chain reaction amplified apo B cDNA, demonstrated 50-56% unmodified (apo B100) mRNA in control and hypothyroid animals and this proportion was unaltered by GH (61% B100 mRNA), despite a reduction in apo B100 synthesis. Both T3 analogs altered apo B mRNA editing (12-16% B100 mRNA) and no apo B100 synthesis was detectable in vivo. Additionally, both T3 analogs produced a 4- to 10-fold induction in hepatic apo A-I and A-IV mRNA abundance, similar to the effects of native T3. GH produced no alteration in apo A-I or A-IV mRNA abundance and neither T3 analog, GH, or native T3 produced a change in apo C III mRNA abundance.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Intestinal apolipoprotein A-IV gene expression in the piglet.

Fetal, newborn, and suckling piglets were used to study the intestinal expression of the apoA-IV gene in the immature mammal. Swine apoA-IV (42 kD) was isolated from fat-fed piglet lipoprotein-deficient plasma by adsorption to Intralipid followed by preparative sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and electroelution. Rabbit anti-swine apoA-IV antibodies were raised, and apoA-IV was immunoprecipitated from small intestinal homogenates after in vivo radiolabeling with [3H]leucine. ApoA-IV synthesis was expressed as a percentage of total protein synthesis from trichloroacetic acid-precipitable counts. Fetal (40 day gestation) whole small intestine synthesis was 2.1%. Postnatally, 2-day-old newborn piglets given high triglyceride and low triglyceride duodenal infusions, as well as bile diversion, were studied. Synthesis rates in jejunal mucosa in all groups were comparable to the fetal whole intestinal value except in the jejunum of the high-triglyceride group, where synthesis was increased sevenfold. In 1- to 2-week-old fasting, cream-fed, and bile-diverted piglets synthesis was again unchanged except in the fat-fed jejunum, where synthesis doubled. Ileal synthesis rates in newborn and suckling animals were lower than jejunal rates and did not increase with lipid absorption or decrease with bile diversion. Northern blot hybridization of intestinal RNA samples from the newborn groups with an authentic cross-hybridizing human apoA-IV cDNA probe revealed a 1.8 kb signal which was strongest in the high-triglyceride jejunal samples. Slot blot hybridization showed eightfold increased apoA-IV mRNA levels in high-triglyceride jejunal samples as compared to low-triglyceride and bile-diverted jejunum with no differences in beta actin mRNA abundance.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Modulation of apolipoprotein B-100 mRNA editing: effects on hepatic very low density lipoprotein assembly and intracellular apoB distribution in the rat.

We have studied the consequences of alterations to hepatic apoB mRNA editing on the biosynthesis and intracellular distribution of newly synthesized apoB variants together with their mass distribution in nascent Golgi very low density lipoproteins (VLDL). Radiolabeled liver membrane fractions were prepared from control or hypothyroid animals and separated by discontinuous sucrose gradient centrifugation. Hepatic apoB-100 synthesis in these groups accounted for 93-100% of total newly synthesized apoB species of Golgi fractions recovered from the sucrose gradients (G1 and G2). The analogous fractions isolated from the livers of hyperthyroid (treated with 3,3',5-triiodo-L-thyronine, T3) animals revealed that newly synthesized apoB-100 accounted for only 46 +/- 10% (G1) and 24 +/- 11% (G2), respectively, of total newly synthesized apoB. ApoB-100 mass in nascent Golgi VLDL from control and hypothyroid G1 fractions represented 70-78% total apoB as determined by Western blot analysis. By contrast, Golgi VLDL from hyperthyroid animals contained predominantly (greater than 78%) apoB-48 as the apoB species. Electron microscopy revealed that the morphology and size distribution of hyperthyroid G1 VLDL were similar to particles isolated from control animals. Thus, despite a profound reduction in the proportion of apoB-100 mRNA species containing an unmodified codon (CAA, B-GLN) at position 2153 in hyperthyroid animals (6 +/- 1% vs 50-61% in control and hypothyroid animals) apoB-100 biosynthesis was detectable in a defined membrane fraction isolated by discontinuous sucrose gradient centrifugation. However, no apoB-100 synthesis was detectable in liver samples prepared by Polytron disruption in Triton-containing buffers. These data suggest that effective hepatic VLDL assembly and secretion in the T3-treated rat continues despite a profound reduction in apoB-100 biosynthesis and implies that apoB-48 contains the requisite domains to direct this process, a situation analogous to that in the intestine.

Animals↗

PubMed1989

Intestinal apolipoprotein synthesis and secretion in the suckling pig.

The present studies report characterization of intestinal apolipoprotein (apoLp) synthesis and secretion in the suckling pig. Lipoproteins (d less than 1.006 g/ml) from mesenteric lymph were found to contain both apoB-100 and B-48, in addition to apoA-IV, E, A-I, and Cs. Lymph low density lipoproteins (LDL) and high density lipoproteins (HDL) contained mainly apoB-100 and apoA-I, respectively. Analysis of core cholesteryl ester fatty acid composition suggested filtration from plasma as the major source of lymph LDL and HDL. Dual radioisotope labeling of intestinal and hepatic apoLps in lymph, as well as immunoprecipitation of radiolabeled intestinal mucosa, demonstrated intestinal synthesis of apoB-48, A-IV, and A-I. There was no evidence for apoB-100 synthesis by intestinal mucosa. By contrast, piglet liver synthesized apoB-100, E, A-I, and Cs, but not apoB-48. Newly synthesized intracellular intestinal apoA-I was mainly (basic) isoform 1 (pI 5.58), while lymph and plasma HDL apoA-I were predominantly isoform 3 (pI 5.33), mature apoA-I. Lymph apoB (P less than 0.001) and apoA-I (P less than 0.04) mass output increased significantly during lipid absorption. Studies were subsequently conducted in fasting, fat-fed, bile-diverted, and sham-operated animals to determine the role of both dietary and biliary lipid in regulating intestinal apoLp biosynthesis. Proximal and distal small intestinal loops were pulse-radiolabeled with [3H]leucine, and apoB-48 and A-I were immunoprecipitated from cytosolic supernatants. Although a proximal to distal gradient in intestinal synthesis rates for both apoB and A-I was noted in all groups, the acute absorption of dietary lipid did not significantly increase apoB or A-I synthesis in either location. Complete removal of biliary lipid for 48 hr did not alter synthesis rates in jejunum or ileum. These studies suggest that mesenteric lymph apoLps in the suckling pig are derived both by filtration from plasma and by direct secretion from the intestine. Physiologic regulation of intestinal apoA-I and B-48 synthesis rates appears to be independent of luminal lipid availability.

Animals↗

PubMed1988

Thyroid hormone modulates the introduction of a stop codon in rat liver apolipoprotein B messenger RNA.

Apolipoprotein B (apoB) biosynthesis by rat liver was studied following thyroid hormone (3,5,3'-triiodo-L-thyronine) administration to hypothyroid rats. Pharmacologic doses of 3,5,3'-triiodo-L-thyronine caused suppression of apoB100 synthesis but did not affect apoB48 levels. There was no detectable apoB100 synthesis in hyperthyroid rats. To examine whether these results were mediated by the previously demonstrated mechanism of RNA modification (Powell, L. M., Wallis, S. C., Pease, R. J., Edwards, Y. H., Knott, T. J., and Scott, J. (1987) Cell 50, 831-840), the DNA sequence corresponding to the C-terminal end of rat apoB48 was determined from rat liver cDNA clones. Rat cDNAs contained a stop codon at an identical position to that found in human and rabbit apoB48 intestinal cDNA. To quantitate the relative amounts of apoB100 and apoB48 message, cDNA was synthesized from hepatic and intestinal apoB RNA and a 207-base pair fragment amplified using the polymerase chain reaction. The products were then differentially hybridized with oligonucleotides specific for apoB100 (containing CAA) or apoB48 (TAA). Control and hypothyroid liver contained approximately equal amounts of CAA and TAA, while hyperthyroid liver contained greater than 90% TAA. All gut samples contained 94-98% TAA. Genomic DNA from rat liver contained only CAA. The results demonstrate that apoB mRNA modification can be hormonally modulated in the adult rat by induction of a mechanism involving substitution of a stop codon into hepatic apoB100 mRNA.

Animals↗

PubMed1988

Rat intestinal apolipoprotein B gene expression. Evidence for integrated regulation by bile salt, fatty acid, and phospholipid flux.

We previously reported that intestinal apo B48 synthesis in the rat was unaltered by dietary triglyceride intake but demonstrated regulation in response to biliary lipid availability. Studies are now presented in which the mechanisms underlying biliary lipid dependent expression of intestinal apo B48 synthesis have been investigated further. Bile salt replacement was effective in a dose- and structure-dependent manner in reexpressing intestinal apo B48 synthesis after prolonged bile diversion. Further experiments suggested that this effect of bile salt may be related to facilitated uptake of fatty acid. A role for mucosal phospholipid flux was suggested by studies in which infusion of lysolecithin, with or without Na taurocholate, produced complete reexpression of apo B48 synthesis in jejunal enterocytes. Over a four- to sixfold range of apo B48 synthesis rates in both jejunum and ileum, there was no change in apo B mRNA size or abundance as determined by RNA blot hybridization. Analysis of both intestinal mucosa and microsome lipid content in a variety of settings revealed that apo B48 synthesis rates were correlated with microsome triglyceride fatty acid content (r = 0.65, P less than 0.005) but not free fatty acid or phospholipid content. These studies demonstrate a physiologic role for elements of biliary lipid flux in the regulation of apo B gene expression. The data suggest that an integrated mechanism may exist whereby apo B48 synthesis is related to microsome triglyceride flux, particularly at low levels of lumenal substrate availability.

Animals↗

PubMed1988

Apolipoprotein gene expression in the rat is regulated in a tissue-specific manner by thyroid hormone.

We have studied the regulation of rat intestinal and hepatic apolipoprotein gene expression, in vivo, after alterations in thyroid hormone status. When compared to those of chow-fed controls, rates of synthesis of intestinal apoA-I and apoB-48 decreased 60-66% in hypothyroid animals and increased three- to fourfold after triiodothyronine (T3) administration. These changes were not accompanied by changes in mRNA abundance. By contrast, intestinal apoA-IV synthesis rates and mRNA abundance were both unaltered over the range of thyroid hormone manipulations tested. Hepatic apoA-I and apoA-IV synthesis rates decreased by 70-80% in hypothyroid animals, while synthesis rates and mRNA abundance increased coordinately six- to eightfold when hypothyroid rats were made hyperthyroid. Hepatic apoE synthesis rates increased twofold in hypothyroid rats and decreased sevenfold in hyperthyroid animals. ApoE mRNA abundance, however, was comparable in all groups. Hypothyroid animals had reduced synthesis rates of hepatic apoB-100 and apoB-48. After induction of hyperthyroidism, apoB-100 synthesis (studied from 5 to 60 min) was undetectable (less than 0.01%) without further change in apoB-48 synthesis and without alterations in either apoB mRNA abundance or transcript size. Despite undetectable hepatic apoB-100 synthesis rates in hyperthyroid animals, total plasma triglyceride secretion rates (after Triton WR-1339 injection) were normalized compared to a 50% decrease in hypothyroid rats. Taken together, the data provide evidence for tissue-specific, independent regulation of apolipoprotein gene expression in vivo. Furthermore, the data suggest that aspects of hepatic triglyceride assembly and secretion and apolipoprotein gene expression may be coordinately responsive to alterations in thyroid hormone status.

Animals↗

PubMed1987

Apolipoprotein A-IV synthesis in rat intestine: regulation by dietary triglyceride.

Apolipoprotein A-IV (apoA-IV) synthesis rates were measured in vivo in rat enterocytes by immunoprecipitation after administration of [3H]leucine into in situ loops of jejunum and ileum. Basal apoA-IV synthesis rates (percent total protein synthesis) were significantly higher in jejunal enterocytes (2.05 +/- 0.54%) compared with ileal enterocytes (0.48 +/- 0.32%) from the same fasted animals. After an acute triglyceride bolus, significant and sustained elevations of apoA-IV synthesis rates were seen in both jejunal and ileal enterocytes with maximal effects noted at 4-6 h. Animals fed diets containing 30% wt/wt triglyceride as saturated (SF) or polyunsaturated (UF) fats for 6 wk had similarly increased rates of apoA-IV synthesis in jejunal enterocytes with both SF (3.73 +/- 0.83%) and UF (3.33 +/- 0.64%) but no change in ileal enterocytes. By contrast, animals consuming a fat-free diet for 3 wk had jejunal apoA-IV synthesis rates indistinguishable from basal values (2.40 +/- 0.45%). Translatable intestinal mRNA levels for pre-apoA-IV after triglyceride increased in parallel to synthesis rates with a 50% increase in jejunum and a 350% increase in ileum observed at 4-6 h. These results suggest that apoA-IV synthesis by rat small intestine increases in response to acute and chronic dietary triglyceride, is maintained in the absence of dietary triglyceride, and may be under pretranslational control.

Animals↗

PubMed1987

Intestinal apolipoprotein A-I and B-48 metabolism: effects of sustained alterations in dietary triglyceride and mucosal cholesterol flux.

In recent studies (1985. J. Lipid Res. 26:368-379 and 1986. J. Lipid Res. 27:30-39) we characterized aspects of synthesis of rat intestinal apolipoproteins (apo) A-I and B-48 in vivo, and their short term regulation by dietary and biliary lipid flux. We now report studies extending these observations to the effects on intestinal apoA-I and apoB-48 metabolism of sustained (3 or 6 weeks) isocaloric intake of diets containing 0-30% (by weight) triglyceride, the latter as either butter fat (saturated) or corn oil (polyunsaturated). Additional studies were conducted to determine, separately, the effects of perturbations of intestinal mucosal cholesterol flux and hypothyroidism on intestinal apoA-I and apoB-48 metabolism. Intestinal synthesis (% total protein) of apoA-I and apoB-48 was not influenced by either dietary triglyceride quantity or quality (saturated vs. polyunsaturated fat); the values that were obtained were strictly comparable to those of both chow-fed animals and animals maintained for 3 weeks on fat-free chow. Intestinal apoA-I synthesis was not influenced by either acute or chronic perturbations of mucosal cholesterol flux. Hypothyroid rats demonstrated a 50% suppression of jejunal apoA-I synthesis. Intestinal synthesis of apoB-48, by contrast, appeared to undergo regulation by chronic (but not acute) perturbations of mucosal cholesterol flux. Maneuvers that augmented intestinal cholesterol uptake (particularly hypothyroidism) appeared to suppress intestinal apoB-48 synthesis by over 40%, while Surfomer (AOMA) administration reduced cholesterol absorption (control, 54 +/- 7%; AOMA, 26 +/- 8%; P less than 0.0005) and resulted in a 24% increase in apoB-48 synthesis by jejunal enterocytes. Intracellular intestinal lipoproteins demonstrated marked cholesteryl ester enrichment of the triglyceriderich lipoprotein fractions in hypercholesterolemic, hypothyroid rats. When all the groups were compared, cholesterol absorption (used as an index of mucosal cholesterol uptake) was negatively correlated with jejunal apoB-48 synthesis (r = -0.92, P less than 0.05). The data suggest that regulation of rat intestinal apoA-I and apoB-48 metabolism is independent of triglyceride flux. It is further concluded that an important regulatory effect of mucosal cholesterol flux can be demonstrated on enterocyte apoB-48 synthesis. Finally, the data suggest the additional possibility that circulating levels of thyroid hormone may exert an independent effect on the expression of rat intestinal apolipoproteins A-I and B-48.

Animals↗

PubMed1986

Apolipoprotein B synthesis in rat small intestine: regulation by dietary triglyceride and biliary lipid.

Apolipoprotein B (apoB) synthesis rates have been determined, in vivo, in rat enterocytes. Following intralumenal administration of a pulse of [3H]leucine, newly synthesized apoB was quantitated by specific immunoprecipitation and compared to [3H]leucine incorporation into total, trichloroacetic acid-insoluble protein. ApoB synthesis rates were determined after acute administration of either 0.1 or 1 g of triglyceride to fasting animals. No differences were found at any time from 90 min to 6 hr after challenge and values were not different from the basal values established in fasted controls. Animals rechallenged with triglyceride after 8 days' intake of fat-free chow also failed to demonstrate a change in intestinal apoB synthesis rate. By contrast, enterocyte content of apoB appeared to fall, temporarily, with the onset of active triglyceride flux. Groups of animals were then subjected to external bile diversion for 48 hr, a maneuver designed to remove all lumenal sources of lipid. Jejunal apoB synthesis rates fell by 43% (from 0.76% +/- 0.14 to 0.43% +/- 0.12, P less than 0.001), a change that was completely prevented by continuous replacement with 10 mM Na taurocholate. The suppression of jejunal apoB synthesis, induced by prolonged bile diversion, was reversed after 14 hr, but not 8 hr, of intralumenal perfusion with 10 mM Na taurocholate. The addition of micellar fatty acid-monoolein to the perfusate for 4 hr produced no further change in apoB synthesis. Ileal apoB synthesis rates fell by 70% (from 0.61% +/- 0.15 to 0.18% +/- 0.10, P less than 0.001) following 48 hr external bile diversion, a change that was only partially prevented by continuous bile salt replacement. These results suggest that jejunal apoB synthesis demonstrates bile salt dependence but not regulation by acute triglyceride flux. The data further suggest that key aspects of the regulation of apoB synthesis by cellular lipid flux may be mediated independently in jejunal and ileal enterocytes.

Animals↗

PubMed1986

Bile acid production in human subjects: rate of oxidation of [24,25-3H]cholesterol compared to fecal bile acid excretion.

Bile acid production has been quantitated in seven subjects by methods that compare the results of two independent approaches, namely, quantitation of cholesterol side-chain oxidation and fecal bile acid excretion. Six hypertriglyceridemic (HT) subjects and one normolipidemic control were studied by both techniques. A further control subject was studied by the cholesterol side-chain oxidation method alone. Cholesterol side-chain oxidation was quantitated by measuring the appearance of 3H2O after intravenous administration of [24,25-3H]cholesterol, using multicompartmental analysis of plasma cholesterol and [3H]water specific activity. Body water kinetics were independently defined by use of oral D2O. Two HT subjects were restudied while they were taking cholestyramine, 16 g/day. In all ten studies, multicompartmental analysis closely simulated the observed appearance of 3H2O. Values obtained for bile acid production suggest that cholesterol oxidation, or bile acid input, was significantly greater than fecal bile acid output in the HT subjects (P less than 0.05). Cholesterol side-chain oxidation rates in the two normal subjects were lower than those encountered in HT subjects, being similar to published values for normal subjects both for bile acid synthesis as determined by isotope dilution kinetics and fecal bile acid excretion. Studies conducted with two, synthetically different, preparations of [24,25-3H]cholesterol indicated that, in one of the two preparations, approximately 20% of the tritium label was at positions proximal to C24. In the other preparation examined, all of the tritium was located at, or distal to, C24. Further studies revealed that 0.055-0.24% of the dose was present as labile tritium by virtue of its appearance as 3H2O following in vitro incubation with human plasma. Provided these isotope effects are taken into account, multicompartmental analysis of plasma [24,25-3H]cholesterol and body water appears to be a useful technique for quantitating cholesterol oxidation in human subjects.

Adult↗

PubMed1985

Variations in dietary triacylglycerol saturation alter the lipid composition and fluidity of rat intestinal plasma membranes.

Rats were maintained on nutritionally complete diets enriched in unsaturated (corn oil) or saturated (butter fat) triacylglycerols. After 6 weeks, significant differences in the lipid composition and fluidity of a number of intestinal membranes were observed. The corn oil diet (enriched mainly in linoleic acid) increased the overall unsaturation of the acyl chains and enhanced the lipid fluidity, as assessed by the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene, of enterocyte microvillus and basolateral membranes and of colonocyte basolateral membranes. Concomitantly, the cholesterol content and the cholesterol/phospholipid molar ratio were increased in the microvillus but not in the basolateral membranes. The increased cholesterol in ileal microvillus membranes can result from enhanced cellular biosynthesis, since ileal slices from rats fed the unsaturated diet incorporated [14C]octanoate more rapidly into digitonin-precipitable sterol. Increased fluidity of the enterocyte microvillus and basolateral membranes, respectively, enhanced the enzyme specific activities of p-nitrophenylphosphatase and (Na+ + K+)-dependent adenosine triphosphatase. The results indicate that the lipid composition, fluidity and enzyme activities of intestinal plasma membranes can be altered by dietary means. Moreover, rat enterocytes possess regulatory mechanisms which modulate the cholesterol content of the microvillus membranes so as to mitigate changes in lipid fluidity.

Animals↗

PubMed1985

Apolipoprotein A-I synthesis in rat small intestine: regulation by dietary triglyceride and biliary lipid.

Techniques were developed to provide direct quantitation of apolipoprotein A-I (apoA-I) synthesis rates in rat small intestine. Following intralumenal administration of a pulse of [3H]leucine, newly synthesized enterocyte apoA-I was quantitated by specific immunoprecipitation and compared to [3H]leucine incorporation into total trichloroacetic acid-precipitable protein. ApoA-I synthesis rates (% total protein) were found to be significantly higher in jejunal enterocytes (1.84 +/- 0.20) compared to ileal enterocytes (0.91 +/- 0.25) from the same, fasting, animals, P less than 0.01. It was found that rats consuming regular (4.5% w/w fat) rodent chow had apoA-I synthesis rates, 30 to 240 min after receiving an intraduodenal bolus of 100 mg of triglyceride, that were indistinguishable from control animals receiving either saline or an isocaloric, but fat-free, enteral preparation. By contrast, animals consuming a fat-free chow for 8 days prior to study had a small but significant response to acute reintroduction of dietary triglyceride. Four hours after 100 mg of triglyceride was administered, jejunal apoA-I synthesis (% total protein) was 1.84 +/- 0.1 compared to 1.37 +/- 0.04 for animals exposed to an isocaloric, fat-free enteral preparation, P less than 0.01. External bile diversion for 48 hr, which effectively removed all lumenal sources of lipid, reduced apoA-I synthesis in jejunal enterocytes but produced no more depression than that found in sham-operated controls infused for 48 hr with dextrosesaline or control animals fasted for 30 hr. By contrast, apoA-I synthesis in ileal enterocytes was reduced significantly by external bile diversion (0.59 +/- 0.20) in comparison to sham-operated controls (1.19 +/- 0.32) P less than 0.01. Continuous infusion of 10 mM Na taurocholate for 48 hr or 10 mM Na taurocholate for 44 hr and 80 mg of micellar lipid for 4 hr produced results similar to those obtained by bile diversion alone (0.56 +/- 0.2 and 0.61 +/- 0.25, respectively) suggesting that bile salt deficiency alone was not responsible for the observed depression in ileal apoA-I synthesis. These results suggest that, under conditions of physiological dietary triglyceride intake, apoA-I synthesis in jejunal enterocytes is not actuely regulated by changes in triglyceride flux. After prolonged dietary triglyceride withdrawal, the reintroduction of fat produces a small, but significant, increase in jejunal apoA-I synthesis. The data further suggest that apoA-I synthesis in jejunal enterocytes is regulated in part by the availability of lumenal lipid, but that the presence of bile does not exert an additional level of control.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1983

Treatment of familial hypercholesterolemia by portacaval anastomosis: effect on cholesterol metabolism and pool sizes.

Measurements of the key parameters of cholesterol homeostasis and the mass of the body pools of cholesterol were carried out in two patients with familial hypercholesterolemia (FH), one homozygote and one heterozygote, before and 28 and 18 months, respectively, after portacaval anastomosis (PCA). In both patients the procedure significantly reduced the plasma concentrations of total and low density lipoprotein cholesterol and the daily rate of whole body cholesterol and bile acid synthesis. In addition, PCA caused a net efflux of accumulated tissue cholesterol as demonstrated by reductions in the rapidly exchangeable and total exchangeable masses of body cholesterol. Shunt patency was verified by demonstration of increased bile acids in serum from fasting patients and from patients 2 hr after a meal and by increased plasma glucagon before and after arginine infusion. Other than a persistently increased level of serum alkaline phosphatase, liver function tests have fallen within the normal range in both patients; there has been no evidence of hepatic encephalopathy. In the homozygous patient there has also been a striking resolution in xanthoma size and distribution. These multiple effects on cholesterol homeostasis and pool sizes strongly suggest that PCA can reverse the progressive accumulation of cholesterol in body tissues of FH patients.

Bile Acids and Salts↗

PubMed1981

Measurement of bile acid production in hyperlipidemic man: does phenotype or methodology make the difference?

Bile acid production has been measured in 13 studies in 10 hyperlipidemic subjects by simultaneous use of isotope dilution kinetics and chemical balance methodology. When the data of all 13 studies were averaged, the correlation between the two sets of values was negative and weak (r - 0.01, NS). However, when the correlations for normoglyceridemic and hyperglyceridemic subjects were examined separately, a strong positive correlation was found between the values obtained by the two methods in normoglyceridemic subjects (r + 0.92, P less than 0.01) but not in hyperglyceridemic subjects (r - 0.25, NS). Examination of bile acid specific activity decay characteristics in eight studies, where bile was sampled up to six times within the first 24 hr after radio-labeled bile acid infusion, revealed differences in three rate of attainment of peak specific activity and in the time taken subsequently to achieve first order kinetic decay. However, analysis of the data from these eight studies by input-output analysis yielded values for primary bile acid synthesis no different that those generated by conventional isotope dilution kinetics. Thus, bile acid production in normoglyceridemic subjects may be accurately quantitated by either isotope dilution or chemical balance methodology. Our data, as well as results from other laboratories, indicate that the values obtained are strictly comparable. On the other hand, the quantitation of bile acid production in hyperglyceridemic subjects by isotope dilution kinetics gives higher values than those obtained by chemical balance methodology, and in addition, higher values than those obtained in patients with normal plasma triglyceride levels.

Adult↗

PubMed1980

Unreliability of tritiated cholesterol: studies with [1,2-3H]-cholesterol and [24,25-3H]cholesterol in humans.

Over the past 18 months different lots of [1,2-3H]cholesterol and [24,25-3H]cholesterol were found to be radiochemically acceptable by conventional chemical and other in vitro tests, yet, when co-administered with [4-14C]cholesterol to human subjects, an abrupt fall in the 3H/14C specific activity ratio in plasma cholesterol was discovered in every case. We have concluded that all batches of [3H]cholesterol should be regarded as radiochemically unreliable unless they are shown to behave identically in all respects to [4-14C]cholesterol in an appropriate in vivo assay system.

Carbon Radioisotopes↗

PubMed1980

In vitro cholesterol synthesis in freshly isolated mononuclear cells of human blood: effect of in vivo administration of clofibrate and/or cholestyramine.

The rate of incorporation of [2-14C]acetate into cholesterol has been measured in freshly isolated peripheral blood mononuclear leukocytes from patients on various hypolipidemic drugs that affect whole body cholesterol synthesis. These studies have demonstrated a significant two-fold increase in mononuclear cell cholesterol synthesis rates in patients receiving cholestyramine, a response measurable after 10 days of drug treatment. Mononuclear cell cholesterol synthesis rates were also measured in four groups of patients on the following drug regimens: 1) no medication, 2) clofibrate (2 g/day), 3) cholestyramine (16 g/day) or 4) both clofibrate and cholestyramine. The results demonstrated that the rate of acetate incorporation into cholesterol was significantly greater in the mononuclear cells from patients receiving either cholestyramine (P less than 0.005) or clofibrate plus cholestyramine (P less 0.001), as compared to controls. Patients receiving clofibrate alone did not differ significantly from controls in their rates of mononuclear cell cholesterol synthesis. Factors other than plasma lipoprotein and lipid levels appeared to be responsible for the elevated sterol synthesis rates observed in all patients receiving cholestyramine.

Cell Separation↗

PubMed1980

Cholesterol absorption in man: effect of administration of clofibrate and/or cholestyramine.

Cholesterol absorption measurements were carried out in a free-living out-patient population by a plasma isotope-ratio method previously validated for in-patients (Samuel, P., J. R. Crouse and E. H. Ahrens, Jr., 1978. J. Lipid Res. 19: 82-93). To test the reproducibility of the method in out-patients, 18 patients were tested twice: the mean intra-assay variability was +/- 6.0%. The method was then applied in 150 hyperlipidemic male out-patients, ingesting a standardized diet containing 250mg cholesterol per day, who had been randomized into four different drug-treatment groups: 1) no medication, 2) clofibrate, (2g/day), 3) cholestyramine (16g/day), or 4) both clofibrate and cholestyramine. Cholesterol absorption (as percent of the oral dose) was increased in patients receiving cholestyramine (P < 0.02) and decreased in those receiving clofibrate (P < 0.02); the group on the combined medication had the same pecent absorption as the control group. In twelve patients receiving cholestyramine, a second test of cholesterol absorption was performed 30 min after each patient had received 8g of cholestyramine. The pre-test administration of cholestyramine caused a 38% decrease in cholesterol absorption (P < 0.001), compared to results obtained when medication was withheld prior to testing. These results demonstrate that the isotope-ratio method of measuring cholesterol absorption is a reproducible procedure applicable to a free-living out-patient population, and that the hypolipidemic drugs, clofibrate and cholestyramine, significantly affect cholesterol absorption in man. The data also show that the results of measurements of cholesterol absorption can be profoundly altered by the type and timing of medication in relationship to the test meal of labeled cholesterol.

Absorption↗