| Abstract: | The unidirectional influx of L-[14C]ascorbic acid (vitamin C) across the mucosal border of guinea pig and human ileum was determined. Influx follows saturation kinetics, indicating that a carrier mechanism is operative. The maximal influx in guinea pig ileum bathed in Ringer is 140 nmol/cm2-h and the ascorbic acid concentration greater than 6 mM , or when the tissue is bathed in Na-free media, influx is approximately linearly related to the ascorbic acid concentration, and absorption by simple diffusion may predominate. With mucosal ascorbic acid concentration of 0.28 mM, influx is reduced by at least 70% when Na-free media is used; however, influx is not dependent on the intracellular Na concentration. The brush border mechanism appears to be stereospecific and not closely coupled to cellular metabolism. A model of transport is favored that features a carrier-mediated transport mechanism for simultaneous entry of ascorbic acid and Na across the brush border. This model is similar in nature to, but functionally distinct from, the sodium-gradient mechanism postulated to effect sugar and amino acid transport in mammalian ileal mucosa. |
