Sterol metabolism. 3. The distribution and transport of sterols across the intestinal mucosa of the guinea pig

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Abstract
The distribution of total sterols and 7-dehydrocholesterol among the components of intestinal-mucosal cells of young fasting female guinea pigs and animals actively absorbing cholesterol and 7-dehydro-cholesterol was studied. In the fasting animal, the sterols are associated with each of the fractions examined: cell membranes and nuclei, mitochondria, microsomes, groundplasm and fatty globules (F-layer). The microsomes contained about 40% of the total in the cell, the mitochondria about 20% with the remainder distributed equally between the cell membranes and nuclei fraction and the groundplasm. The amount in the F-layer was less than 0.5%. The sterols undergoing absorption were observed to enter each fraction but the number of results obtained was insufficient to determine unequivocally if the uptake represented an absolute increase in each fraction or merely an exchange between the dietary and endogenous molecule. The distribution of 7-dehydrocholesterol in relation to total sterols and of fast-reaction to slow-reaction sterols was uniform across the mucosal cells of the fasting animals and tended towards uniformity in the dosed animals. These findings imply a rapid exchange of the dietary with the endogenous sterols during the passage of the former across the intestinal mucosa. The above evidence leads to the conclusion that the absorption of the sterols takes place at the molecular level via a rapid exchange and transfer process between the various lipoproteins of the cell membrane, organelles and groundplasm. An explanation for the poor absorption of phytosterols and their inhibition of cholesterol absorption in relation to the above hypothesis is discussed. Evidence was obtained for the reduction of 7-dehydrocholesterol to lathosterol during the absorption of the former across the intestine.