Quantitation of the enterohepatic circulation of retinol in the rat

Studies were done to determine whether vitamin A is reabsorbed in the enterohepatic circulation in forms that can be reutilized by the body for vision and reproduction. Micelles containing [11,12(n)-3H]retinol were administered orally to mesenteric lymph duct-cannulated rats. Lymph containing [3H]retinyl esters and small amounts of [3H]retinol and [3H]retinaldehyde was collected from these rats and an aliquot was injected into the jugular vein of a second group of mesenteric lymph duct-cannulated rats. The total radioactivity recovered in 24-h lymph collections from recipient rats was only 1.6% of that injected, and the [3H]retinyl esters, [3H]retinol and [3H]retinaldehyde recovered was only 0.9% of that injected. Thus, the amount of vitamin A recovered in the enterohepatic route in forms that can be reutilized by the body for vision and reproduction appears to be very small. Notable differences were observed in the composition of mesenteric lymph containing newly absorbed vitamin A and mesenteric lymph containing recycled vitamin A. In the former, 87% of the total lymph radioactivity was associated with retinyl esters, 5% with retinol, 3% with retinaldehyde and 5% with unidentified mostly nonpolar compounds. In the latter, only 41% of the total lymph radioactivity was associated with retinyl esters, 12% with retinol, 3% with retinaldehyde and 44% with unidentified mostly polar compounds.