Intestinal absorption and metabolism of ascorbic acid in rainbow trout.

Transport and metabolism of the reduced and oxidized forms of vitamin C were evaluated in trout intestine in vitro. Transepithelial fluxes of [14C]ascorbic acid were determined with the substrate present at 10 microM. The mucosa-to-serosa flux was fourfold greater than the serosa-to-mucosa flux with ascorbic acid present in both bathing solutions. Cellular accumulation of the reduced molecule occurred with a final tissue-to-medium ratio of 1.64. Ascorbic acid influx across the brush-border membrane was determined under conditions that inhibit transport of other nutrients. Influx was not reduced because of the presence of the D- or L-stereoisomeric form of glucose but was highly dependent on the presence of Na in the bathing medium. Brush-border influx was saturable with a Km of 220 microM. Short-term (8 min) exposure of intestinal loops to the oxidized form of vitamin C, dehydro-L-ascorbic acid, was followed by high-performance liquid chromatography analysis of the 14C label present in the tissue. Total uptake of substrate was greater from the serosal solution than from the mucosal solution; in either situation most of the 14C label was found in the reduced state. It is concluded that transport and metabolic capabilities are present in the trout intestine to absorb oxidized and reduced forms of vitamin C and to help regulate the redox potential of circulating ascorbic acid by recycling dehydro-L-ascorbic acid.