<i>In vivo </i>Plasma Cholesterol Exchanges in the Digestive Tract of the Rat: Effect of <i>L</i>-Thyroxine

Abstract

Cholesterol exchanges between plasma and digestive organs (stomach, small intestine divided into jejunum and ileum, cecum plus colon) were studied in control and in L-thyroxine-fed rats receiving a constant venous infusion of (³H)-cholesterol for 3 or for 6 h and in unoperated rats by use of the isotopic equilibrium method. In control rats, total cholesterol radioactivity in the walls of stomach, intestine and cecum-colon averaged 0.50% of infused (³H)-cholesterol at the end of 6 h; between 70 and 75% of this radioactivity was located in the intestine. However, the relative transfer rates into these walls, expressed as tissue radioactivity for the same free cholesterol specific activity in the plasma at the moment of sacrifice, are nearly the same when expressed per gram of tissue (walls) or per milligram of DNA in the epithelial cells of the jejunum and the ileum. In thyroxine-fed rats, total cholesterol radioactivity in the walls reached 1.27% of infused (³H)-cholesterol after 6 h, the intestine still containing the major part of radioactivity (80%). The relative cholesterol transfer rates into the walls are unchanged in stomach, ileum and cecum-colon when compared to the controls. But in the jejunum, L-thyroxine causes a twofold increase in plasma cholesterol exchanges through the wall and through the epithelial cell membrane. Moreover, in the epithelial cell of the jejunum, this hormone increases the percentage of plasma cholesterol, as estimated by the isotopic equilibrium method. Plasma cholesterol radioactivity enters the lumen mainly through the jejunal mucosa. L-Thyroxine strongly enhances jejunal excretion of plasma cholesterol explaining the twofold increase of fecal cholesterol excretion.

Anatomical Heterogeneity of the Intestinal Mucosa and Cholesterol Homeostasis

“…In rats receiving daily subcutaneous injec tions of labelled cholesterol for 1 month, the specific activity of cholesterol is similar in top or villus cells from the duodenum or ileum, and reaches 50-55% of plasma cholesterol-specific activity. This shows that about half of the enterocyte cholesterol has been exchanged with plas ma cholesterol in the course of the cell's life [29,30], Since a similar pattern is noted under those conditions, in the crypt and apical cells, infusion experiments were performed in order to label only plasma cholesterol in bile fistula rats receiving a compensatory infusion of (unla belled) bile. Cholesterol-specific activity in enterocytes was monitored during 3 or 6 h. The distribution of plasma cholesterol in the core villus and in the epithelial cells localized at the different levels of the villus was also observed 6 or 11 h after either an intravenous infusion or a pulse of radiolabelled cholesterol [30], These experi ments demonstrated that the exchanges of plasma choles terol through the epithelial basal membrane are lower than through the capillary membrane, and do not depend on the localization of the epithelial cell on the villus (crypt or apex).…”

In vivo cholesterol synthesis by the rat digestive tract. II. A Study of turnover

“…Moreover, cholesterol is exchanged between the plasma, stomach and caecum-colon. These exchanges are slightly lower than for the intestine (Lutton and Brot-Laroche, 1980). These two processes should lead to a progressive decrease in the radioactivity incorporated into cholesterol after 14 Cacetate injection.…”

“…If the model for the turnover of cholesterol synthesized in the intestine is assumed to be an open compartment having 2 outputs, fecal elimination (RE) and transfer into plasma (RI) (fig. 4) (Giraud-D'Hollander et al, 1976 ;Lutton and Brot-Laroche, 1980 (Lutton and Brot-Laroche, 1979). …”

Lymphatic Transport of Cholesterol from Exogenous and Biliary Origins in Nonfasting Rats After Intraduodenal Infusions of Triolein

“…As it is well established that the lymphatic pathway is the exclusive transport route for cholesterol absorbed and that the metabolism of its molecule (degrada tion of the steroid nucleus by the intestinal microflora included) is very slow, this obser vation could be explained by a transfer of cholesterol molecules between enterocytes and plasma [13]: a part of infused labelled cholesterol, after its uptake from the lumen into the intestinal mucosa would not have been released into the lymphatics but ex changed with plasma cholesterol. Unfortu nately our experimental schedule did not al lowed us to recover it.…”

“…The low rates of radioactivity in bile and blood are due to a large dilution of labelled cholesterol with nonlabelled choles terol of the total body, as a simultaneous transfer of cholesterol takes place between the plasma and any tissue [13][14][15][16].…”