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J M Dietschy

Publications and source records attributed to J M Dietschy.

At least 109 records · Page 6Linked to original sources
PubMed1981

Rates of sterol synthesis and uptake in the major organs of the rat in vivo.

This study was undertaken to determine the rates of sterol synthesis and uptake in the major organs of the female rat in vivo. At the mid-dark phase of the light cycle, control animals, animals in which hepatic sterol synthesis had been selectively inhibited by chylomicron infusion and animals in which the small intestine and liver had been surgically removed, were administered [(3)H]water, and the content of (3)H-labeled digitonin-precipitable sterols ([(3)H]-DPS) in different organs was measured 1 hr later. In control animals, the highest content of [(3)H]DPS was found in the liver (2279 nmol/hr per g), adrenal gland (1222), ovary (791), and small bowel (529): the content of newly synthesized sterols was much lower in 13 other tissues. By selectively inhibiting sterol synthesis in the liver or by surgically removing the small intestine and liver, it was determined that of the total amount of [(3)H]DPS synthesized in the whole animal about 50% had occurred in the liver, 24% in the small bowel, 8% in the skin, and 18% in the remaining tissues of the carcass combined. By analyzing the relationship between the content of [(3)H]DPS in blood and in each organ, it was further possible to determine how much [(3)H]DPS was synthesized and how much was taken up from the blood in each tissue. The highest rate of uptake was found in the adrenal gland where only 4% of the tissue content of [(3)H]DPS came from local synthesis. Low rates of synthesis relative to the rates of uptake, were also found in the spleen (6%), lung (17%), and kidney (26%). In contrast, in other organs there was little uptake of [(3)H]DPS from blood so that >75% of the [(3)H]DPS present in brain and muscle, for example, was due to local synthesis. Lowering the circulating levels of plasma cholesterol markedly increased the synthesis of [(3)H]DPS in tissues like adrenal gland, spleen, and kidney that were dependent upon plasma cholesterol as the major source of tissue sterols, but not in tissues such as muscle and brain. These studies have quantitated the importance of each major organ to total body synthesis and have delineated the rates of movement of [(3)H]DPS between major tissue compartments of the rat.-Turley, S. D., J. M. Andersen, and J. M. Dietschy. Rates of sterol synthesis and uptake in the major organs of the rat in vivo.

Animals↗
PubMed1980

Permeability characteristics of muscle membrane.

Unidirectional flux rates of saturated fatty acids, saturated alcohols, and bile acids were measured in an intact rat diaphragm preparation. The logarithm of the permeability coefficients for fatty acids containing from five to ten carbon atoms was a linear function of the number of carbon atoms in the fatty acid chain. Incremental free energies of solution were +336 cal x mol-1 for the addition of a hydroxyl group and -258 cal x mol(-1) for the addition of a methylene group. These incremental free energies were similar to those obtained by other investigators in other animal tissues, and our data suggest a structural similarity between membranes in different tissues and in different species. The muscle membrane exhibited anomalously high permeabilities for fatty acids containing less than five carbon atoms. Since muscle lacks tight junctions, this result suggest that small non-electrolytes traverse polar regions or aqueous pores within the cellular membrane.

Alcohols↗
PubMed1980

Effects of clofibrate, cholestyramine, zanchol, probucol, and AOMA feeding on hepatic and intestinal cholesterol metabolism and on biliary lipid secretion in the rat.

Utilizing newer techniques for measuring rates of cholesterol synthesis, the effects of clofibrate, cholestyramine, zanchol, probucol, and AOMA (a copolymer of maleic acid and an alpha-olefin chain of 18 carbons) on hepatic and intestinal cholesterol metabolism and on biliary lipid secretion in the rat were examined. Uncer conditions of isocaloric feeding, female rats were administered a fixed amount of these drugs each day for 14 days. In one set of experiments the rats were killed and rates of hepatic and intestinal cholesterol synthesis and tissue cholesterol levels were measured. In a second set of studies the rats were subjected to total biliary diversion for 2 hr and rates of biliary lipid secretion were determined. Plasma cholesterol concentrations were measured in all animals. Neither AOMA nor cholestyramine had any effect on plasma cholesterol levels or biliary lipid secretion, but both markedly enhanced intestinal cholesterol synthesis. Cholestyramine also increased hepatic cholesterol synthesis. Clofibrate significantly decreased plasma cholesterol levels, whereas zanchol had the opposite effect. Both clofibrate and zanchol increased liver size and bile secretion rates but lowered biliary lipid concentrations; however, total biliary lipid output was unchanged. Total hepatic cholesterol synthesis in clofibrate-fed animals was similar to that in the controls, but in zanchol-fed animals it was approximately doubled. Neither clofibrate nor zanchol had any effect on intestinal cholesterol synthesis. Probucol dramatically lowered plasma cholesterol levels but had no effect on the other parameters measured. None of the drugs was effective in altering the molar percentage of cholesterol in bile despite the diversity of metabolic changes which occurred. These studies point up the many diverse metabolic effects of these drugs, some of which are clinically useful in lowering plasma cholesterol levels, and delineate the marked dissociations that occur between rates of cholesterol synthesis, tissue cholesterol levels, and plasma cholesterol concentrations.

Animals↗
PubMed1980

Intestinal kinetic parameters: effects of unstirred layers and transport preparation.

Three in vitro tissue preparations were used to derive kinetic parameters for the transport of D-glucose in rabbit jejunum, and the resistance of the unstirred water layer was varied by altering the rate of stirring of the bulk phase. The apparent permeability coefficient (Pd*) of the rabbit jejunum for D-glucose was much higher from everted sacs and full-thickness biopsies than from intestinal discs. Failure to adjust the experimentally determined flux for the contribution of the passive component led to errors in the estimation of the maximal transport rate (Jdm) and in the apparent Michaelis constant (Km*). Jdm was higher in biopsies than everted sacs or discs, Km was also higher in biopsies. With each tissue preparation Km* and Pd* were markedly influenced by stirring the bulk phase, whereas Jdm was unchanged. The results indicate that failure to account for the effect of the passive component and the unstirred layer leads to major errors in the estimation of Km*, Pd*, and Jdm. Furthermore, the magnitude of these kinetic constants is influenced by the type of in vitro system used to derive the constants, and it is therefore invalid to extrapolate the results obtained using one preparation to those utilizing another preparation, or to the in vivo situation.

Animals↗
PubMed1980

Regulation of rates of cholesterol synthesis in vivo in the liver and carcass of the rat measured using [3H]water.

This study was undertaken to determine the mechanisms that regulate cholesterol synthesis in vivo and to quantitate the relative importance of the liver and extrahepatic tissues as sites for sterol synthesis. Rats were administered [3H]water intravenously and killed 1 hour later. The amount of [3H]water incorporated into digitonin-precipitable sterols was then measured in liver, whole blood, and the remaining tissues of the carcass. In control animals, killed at the mid-dark point of the light cycle, rates of [3H]water incorporation into sterols equaled 2290 and 103 nmol/hr per g, respectively, in the liver and carcass. Cholesterol feeding suppressed synthesis in the liver but not in the extrahepatic tissues, while fasting for 48 hr suppressed synthesis in both the liver and carcass. In fasted animals subjected to stress there was a 5-fold increase in hepatic synthesis but no change in synthesis by the extrahepatic tissues. Similarly, incorporation of [3H]water into sterols by the carcass was unaffected by light cycling while the liver showed a definite diurnal rhythm. In control rats, 34.5 mumol of [3H]water was incorporated into sterols by the whole animal per hour. Of this amount of sterol synthesis about 54% took place in the liver while the remaining amount occurred in the tissues of the carcass. With cholesterol feeding or fasting, or during the mid-light phase of the light cycle, synthesis in the extrahepatic tissues accounted for 69 to 90% of total body sterol synthesis.

Animals↗
PubMed1979

Low and high density lipoproteins and chylomicrons as regulators of rate of cholesterol synthesis in rat liver in vivo.

The steady-state levels of plasma cholesterol carried in high and low density lipoproteins and in chylomicrons were varied over a wide range by use of a constant-infusion technique. After 40 hr, the rates of hepatic cholesterol synthesis and levels of hepatic cholesterol esters were measured and were related to the plasma level of each of the lipoprotein fractions. From the rates of infusion and the steady-state plasma levels attained, the whole animal clearance rates for cholesterol carried in low density and high density lipoproteins and in chylomicrons were calculated to be 0.53, 0.61, and 42.6 ml/hr, respectively. Hepatic cholesterol ester content increased by 0.8 mug/g for each 1.0 mg/dl increase in the steady-state level of plasma low density lipoprotein cholesterol and by 1.4 mug/g for a similar elevation in plasma high density lipoprotein cholesterol. In contrast, the increase in ester content was 300-fold greater when chylomicrons were infused (330 mug/g). The rate of hepatic cholesterol synthesis was inhibited by a factor of 0.004 and 0.007, respectively, per 1.0 mg/dl increase in the steady-state level of plasma cholesterol carried in either low density or high density lipoprotein but the inhibition was by a factor of 0.255, 50-fold greater, when chylomicrons were infused. Thus, in the steady state, cholesterol carried in either low density or high density lipoproteins is apparently taken up by the liver and regulates the rate of hepatic cholesterol synthesis; however, cholesterol carried in chylomicrons is at least 50-fold more effective in this regard. This marked difference can be attributed to the much higher rates of transport of chylomicron cholesterol into the hepatocyte than of cholesterol carried in either low density or high density plasma lipoproteins.

Animals↗
PubMed1979

Determinants of the rate of intestinal absorption of oral cholecystographic contrast agents in the dog jejunum.

Successful opacification of the gallbladder during oral cholecystography depends on adequate absorption of the contrast agent from the intestine. The present studies were undertaken to define the specific characteristics of the intestinal mucosa and the properties of the various contrast agents which determine their rates of intestinal absorption. The polarity of the compounds was established by determining the ratios of their distribution between bulk solvents (benzene and water). In addition, the maximum aqueous solubility of each compound was determined. Using in vivo cannulated segments of dog jejunum, apparent passive permeability coefficients were measured. From these data, the apparent maximal rate of intestinal absorption was calculated. The six cholecystopaques studied differed markedly in polarity as judged by their varying ratios of distribution between benzene and water. The permeability coefficients varied inversely with the polarity of the compounds. However, the incremental changes in the coefficients were considerably less than the corresponding changes observed in the partition ratios. The rates of absorption of the more polar contrast agents (tyropanoate, iopronic acid, and iocetamic acid) were greater than the less polar compounds (iopanoic acid, sodium ipodate, and calcium ipodate) under the conditions in which the resistance of the unstirred water layer is not rate limiting and where bile acid micelles are not present.

Animals↗
PubMed1979

Absolute rates of cholesterol synthesis in extrahepatic tissues measured with 3H-labeled water and 14C-labeled substrates.

This study was undertaken to develop techniques for measuring absolute rates of sterol synthesis in extrahepatic tissues in vitro and to estimate the magnitude of the errors inherent in the use of various 14C-labeled substrates for such measurements. Initial studies showed that significant errors were introduced when rates of synthesis were estimated using [3H]water since about 20 nmol of water were bound to each mg of tissue cholesterol isolated as the digitonide. This source of error could be eliminated by subtracting apparent incorporation rates obtained at 0 degrees C from those obtained at 37 degrees C or by regenerating and drying the free sterol. In a second set of experiments, the H/C incorporation ratio in cholesterol was determined in the liver by measuring the absolute rates of hydrogen and acetyl CoA flux into sterols. The ratio of 0.69 +/- 0.03 was found to be independent of the rate of hepatic cholesterol synthesis, the rate of hepatic acetyl CoA generation, or the source of the acetyl CoA. In a third set of studies, rates of incorporation of [3H]water or 14C-labeled acetate, octanoate, and glucose into digitonin-precipitable sterols were simultaneously measured in nine different extrahepatic tissues. Assuming that the H/C ratio measured in the liver also applied to these tissues, the [3H]water incorporation rates were multipled by the reciprocal of the H/C ratio to give the absolute rates of sterol synthesis in each tissue. When these were compared to the incorporation rates determined with the 14C-labeled substrates the magnitude of the errors in the rates of sterol synthesis obtained with these substrates in each tissue could be assessed. Only [14C]octanoate gave synthesis rates approaching 100% of those obtained with [3H]water and this occurred only in the intestine and kidney; in the other extrahepatic tissues this substrate gave rates of 6--66+ of the absolute rates. Rates of [14C]acetate incorporation in sterols varied from 4 to 62% of the [3H]water incorporation rates while those obtained with [14C]glucose were only 2--88% of the true rates. These studies document the large and highly variable errors inherent in estimating rates of sterol synthesis in extrahepatic tissues using 14C-labeled substrates under in vitro conditions.

Acetates↗
PubMed1979

Regulation of biliary cholesterol output in the rat: dissociation from the rate of hepatic cholesterol synthesis, the size of the hepatic cholesteryl ester pool, and the hepatic uptake of chylomicron cholesterol.

These studies were designed to determine the importance of the rate of hepatic cholesterol synthesis, the size of the hepatic cholesteryl ester pool, the amount of chylomicron cholesterol reaching the liver, and the rate of bile acid transport into bile as determinants of the rate of biliary cholesterol output. Female rats that had been subjected to diurnal light cycling, fasting for 48 hr, intravenous administration of chylomicrons, and diets containing either cholestyramine, cholesterol, or bile acid underwent total biliary diversion for 2 hr. The animals were then killed and the rates of hepatic cholesterol synthesis and levels of hepatic esterified cholesterol were measured along with biliary lipid concentrations. Despite a 1000-fold variation in the rate of hepatic cholesterogenesis and a 100-fold variation in the levels of cholesteryl esters, the output and molar percentage of cholesterol in bile remained essentially constant with the exception of an approximate doubling in the output of cholesterol, as well as of bile acid and phospholipid in those animals fed bile acid. However, in this latter group the molar percentage of each component was unchanged. The administration of a bolus of chylomicrons did not alter output or molar percentage of cholesterol. Total biliary diversion for 36 hr and bile acid infusion were used to markedly vary the rate of biliary bile acid output. Cholesterol and phospholipid output remained tightly coupled to bile acid output over almost a 40-fold range. In other experiments it was shown that biliary cholesterol output could be driven by bile acid infusion to a similar extent in rats in which the rate of hepatic cholesterogenesis had been varied over a 26-fold range. It was concluded that the rate of hepatic cholesterol synthesis, the level of hepatic cholesteryl esters, and the amount of cholesterol absorbed from the diet play no role in determining the rate of biliary cholesterol secretion, at least in this species.-Turley, S. D., and J. M. Dietschy. Regulation of biliary cholesterol output in the rat: dissociation from the rate of hepatic cholesterol synthesis, the size of the hepatic cholesteryl ester pool, and the hepatic uptake of chylomicron cholesterol.

Animals↗
PubMed1978

The mechanisms of and the interrelationship between bile acid and chylomicron-mediated regulation of hepatic cholesterol synthesis in the liver of the rat.

Hepatic cholesterol synthesis is controlled by both the size of the bile acid pool in the enterohepatic circulation and by the amount of cholesterol reaching the liver carried in chylomicron remnants. These studies were undertaken to examine how these two control mechanisms are interrelated. When the size of the pool was systematically varied, the logarithm of the rate of hepatic cholesterol synthesis varied in an inverse linear fashion with the size of the taurocholate pool between the limits of 0 and 60 mg of bile acid per 100 g of body weight. The slope of this relationship gave the fractional inhibition of cholesterol synthesis associated with expansion of the taurocholate pool and was critically dependent upon the amount of cholesterol available for absorption from the gastrointestinal tract. Furthermore, the degree of inhibition of cholesterol synthesis in the liver seen with taurocholate feeding was reduced by partially blocking cholesterol absorption with beta-sitosterol even though the bile acid pool was still markedly expanded. In rats with diversion of the intestinal lymph from the blood, a five-fold expansion of the taurocholate pool resulted in only slight suppression of the rate of hepatic cholesterol synthesis, and even this inhibition was shown to be attributable to small amounts of cholesterol absorbed through collateral lymphatic vessels and (or) to a fasting effect. Similarly, the infusion of either taurocholate or a combination of taurocholate and taurochenate into rats with no biliary or dietary cholesterol available for absorption caused no suppression of hepatic cholesterol synthesis. Finally, the effect of changes in the rate of bile acid snythesis on hepatic cholesterol synthesis was examined. The fractional inhibition of cholesterol synthesis found after administration of an amount of cholesterol sufficient to raise the hepatic cholesterol ester content by 1 mg/g equalled only --0.36 when bile acid snythesis was increased by biliary diversion but was --0.92 when bile acid synthesis was suppressed by bile acid feeding. It is concluded that (a) bile acids are not direct effectors of the rate of hepatic cholesterol synthesis, (b) most of the inhibitory activity seen with bile acid feeding is mediated through increased cholesterol absorption, and (c) bile acids do have an intrahepatic effect in that they regulate hepatic cholesterol synthesis indirectly by altering the flow of cellular cholesterol to bile acids.

Animals↗
PubMed1978

Re-evaluation of the 3 alpha-hydroxysteroid dehydrogenase assay for total bile acids in bile.

A review of the 3 alpha-hydroxysteroid dehydrogenase method for determining the concentration of total bile acids in bile is described. The optimum conditions for the assay were established with respect to pH, temperature, incubation time, amount of NAD+, and units of enzyme activity required to obtain complete oxidation of the substrate under fixed conditions. Furthermore, the effect of hydrazine hydrate, methanol, and bile volume on the reaction was examined. It was also established that the bile acid concentration in bile samples with a high molar percentage of cholesterol would be overestimated if 3 beta-hydroxysteroid dehydrogenase were present with the 3 alpha-enzyme.

3-Hydroxysteroid Dehydrogenases↗
PubMed1977

Rate constants for the uptake of cholesterol from various intestinal and serum lipoprotein fractions by the liver of the rat in vivo.

The increase in the mass of cholesterol esters in the liver was used to estimate hepatic net uptake rates of cholesterol from various serum and intestinal lipoprotein fractions. Initial uptake rates equalled essentially zero for high density serum lipoproteins and for large chylomicrons while administration of both low density serum lipoproteins and smaller chylomicrons produced a significant increase in hepatic cholesterol ester levels. The rate of uptake of both serum lipoprotein fractions did not change over a 5 h interval after injection: in contrast, the rates of uptake of the intestinal fractions increased 10-25-fold during this interval. Circulation of large chylomicrons in functionally eviscerated rats markedly increased the rate of hepatic cholesterol uptake when these metabolized lipoproteins were reinjected into recipient animals. Uptake of cholesterol from intestinal lipoproteins was essentially a linear function of the amount of chylomicrons administered to the animals and was independent of the level of circulating serum cholesterol and the rate of hepatic cholesterogenesis. These observations are consistent with the view that the liver is capable of taking up cholesterol from chylomicron remnants and, at significantly lower rate, low density serum lipoproteins.

Animals↗