Vitamin A1 intestinal absorption in vivo: influence of luminal factors on transport.

Intestinal absorption of [3H]retinol was studied in the unanesthetized rat. Luminal perfusate was recirculated through isolated intestinal segments with intact vascular and lymphatic circulation. Apparent saturation kinetics were found in physiological concentrations of retinol, whereas a linear relationship between the concentration and absorption rate was found at pharmacological concentrations of retinol in the perfusate. In physiological concentrations, retinol uptake in vitro by everted gut sacs was unaffected by anoxia or metabolic inhibitors and uncouplers. In vivo retinol absorption rate was decreased when sodium taurocholate concentration was raised above 5 mM, or when 2.5 mM linoleic or linolenic acids were added to the perfusate. Absorption increased markedly as the thickness of the unstirred water layer was diminished. Variations in perfusate pH from 4.5 to 8.6 did not change the retinol absorption rate. In vivo absorption of retinol in physiological concentrations is mediated by a saturable, carrier-mediated passive absorption mechanism modified by the presence of fatty acids of varying chain length.