In vivo plasma cholesterol exchanges in the digestive tract of the rat: effect of L-thyroxine

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 cecumcolon 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-Thyroxyine strongly enhances jejunal excretion of plasma cholesterol explaining the twofold increase of fecal cholesterol excretion.