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J M Dietschy

Publications and source records attributed to J M Dietschy.

At least 163 records · Page 9Linked to original sources
PubMed1969

Failure of bile acids to control hepatic cholesterogenesis: evidence for endogenous cholesterol feedback.

Studies were undertaken to define the role of bile acids in the control of hepatic cholesterogenesis from acetate. Both biliary diversion and biliary obstruction increase the rate of sterol synthesis by the liver 2.5- to 3-fold. After biliary diversion, however, the bile acid content of the liver is decreased, whereas after biliary obstruction, it is markedly increased. Thus, there is no relationship between the tissue content of bile acid and the rate of hepatic cholesterol synthesis. Furthermore, restoration of the enterohepatic circulation of bile acid in animals with biliary diversion fails to prevent the rise in synthetic activity seen after this manipulation. These data indicate that bile acid plays no direct inhibitory role in the regulation of cholesterol synthesis by the liver. Other experiments were therefore undertaken to evaluate the possibility that changes in cholesterogenic activity observed after manipulation of the enterohepatic circulation of bile acid actually are the result of changes in the enterolymphatic circulation of cholesterol. In support of this thesis it was found that intestinal lymphatic diversion causes the same specific enhancement of cholesterol synthetic activity as biliary diversion and that both of these operative procedures increase enzymatic activity at the step mediated by beta-hydroxy-beta-methyl glutaryl reductase. Furthermore, the increase in the rate of sterol synthesis by the liver seen in animals with biliary diversion can be prevented by the infusion of approximately 7 mg of cholesterol/24 hr in the form of chylomicrons. This is an amount of cholesterol circulating normally in the enterolymphatic circulation of the intact rat.These results indicate that bile acid plays no direct role in the control of hepatic cholesterogenesis, but rather, it is the enterohepatic circulation of endogenous cholesterol that determines directly the rate at which cholesterol is synthesized by the liver.

Acetates↗
PubMed1968

Cholesterol synthesis in the squirrel monkey: relative rates of synthesis in various tissues and mechanisms of control.

Cholesterol synthesis has been extensively investigated in various tissues of lower mammals; however, there is little specific information concerning cholesterologenesis in the primate. Furthermore, experiments in whole animals suggest that important differences may exist in the features of cholesterologenesis in the dog and rat versus the monkey and man. Using the new world squirrel monkey, therefore, we performed the present studies to determine the rates of cholesterologenesis in various tissues per unit weight, to define the relative rates of whole organ synthesis, and to evaluate the operation of control mechanisms in these tissues.In control animals fed a low cholesterol chow diet, the liver and ileum were the two most active sites for cholesterologenesis followed, in order, by the colon, esophagus, and proximal small bowel. Rates of synthesis in 10 other tissues tested were considerably lower than these found in the gastrointestinal tract. When rates of whole organ synthesis were calculated, three tissues, i.e., liver, bowel, and skin, accounted for 92% of the total demonstrable synthetic activity.Following cholesterol feeding utilizing either a solid chow or liquid formula diet, marked suppression of hepatic cholesterologenesis occurred while synthesis in other organs remained essentially unaltered. Similarly, fasting animals for periods up to 96 hr resulted in suppression of synthesis in the liver, but not in various levels of the intestine. Finally, biliary diversion for 48 hr caused a twofold increase in hepatic cholesterologenesis and a six- to eightfold increase in sterol synthesis in the small but not the large intestine.

Journal Article↗
PubMed1968

The role of bile salts in controlling the rate of intestinal cholesterogenesis.

According to current concepts, the liver and gastrointestinal tract are considered to be the major, if not the sole, sources of circulating serum cholesterol. While several mechanisms have been described which control the rate of hepatic cholesterogenesis, only biliary diversion is known to alter the rate of sterol synthesis in the intestine. The present study was designed to identify the inhibitory constituent of bile and to define its anatomic and biochemical sites of action. After either biliary diversion or cholestyramine feeding, there is a marked enhancement of cholesterogenesis at every level of the small intestine; this effect is specific for sterol synthesis since acetate incorporation into fatty acids and CO(2) is unaffected by these experimental manipulations. In the present investigation bile salt has been shown to be the constituent of whole bile responsible for the inhibited rate of sterol synthesis found in the intact animal, and in addition, an inverse relationship has been shown to exist between the steady-state intraluminal bile salt concentration and the rate of cholesterogenesis in the adjacent bowel wall. The inhibitory effect of bile salt is directed at the cells of the intestinal crypt, the major anatomic site for sterol synthesis in the small bowel. This feedback inhibition has been localized in the biosynthetic sequence to a step between acetyl CoA and mevalonic acid and, presumably, is at the enzymatic step mediated by hydroxymethylglutaryl reductase. These studies emphasize the close interrelationship which exists between the mechanisms of control of cholesterogenesis in the liver and small intestine. Sterol synthesis in the liver is regulated by exogenous cholesterol intake, whereas the rate of intestinal sterol synthesis is controlled by bile salt, the major end product of the hepatic catabolism of cholesterol.

Acetates↗
PubMed1968

Mechanisms for the intestinal absorption of bile acids.

In this review experimental data are summarized which indicate that at least four different transport mechanisms account for net movement of bile acids across the gastrointestinal tract. These are active transport and the passive mechanisms of ionic, nonionic, and micellar diffusion. Of these four, active transport and passive nonionic diffusion are quantitatively of the greatest importance. Active transport is confined to the ileum and probably plays a dominant role in the absorption of conjugated bile acids. Passive nonionic diffusion may occur at any level of the gastrointestinal tract and probably is the major mechanism for the absorption of unconjugated bile acids.

Animals↗
PubMed1967

Effect of cholesterol feeding and fasting on sterol synthesis in seventeen tissues of the rat.

Rates of sterol synthesis were measured in 17 tissues of the rat, and the responsiveness of these rates to cholesterol feeding and to fasting was determined. The liver and gastrointestinal tract together account for 90% of synthetic activity of the whole body. After the rats had been fed cholesterol or fasted, liver synthesis was markedly decreased, whereas synthetic rates in all other organs tested were essentially unaffected (this conclusion applies to synthesis of cholesterol and of five other digitonin-precipitable tissue sterols). Consequently, the highest rate of cholesterogenesis in the cholesterol-fed or fasted rat is found in the gastrointestinal tract.

Animals↗