Active secretion of hypoxanthine and xanthine by guinea pig jejunum in vitro

Isolated epithelium of guinea pig jejunum secretes hypoxanthine and xanthine by a transport process that is capable of uphill transport and dependent on metabolic energy supply. Unidirectional influx of hypoxanthine across both the luminal and the contraluminal cell membrane appears to be saturable; influx across the contraluminal membrane is inhibited by 2,4-dinitrophenol (DNP). Efflux across the luminal membrane is diminished by DNP; efflux across the contraluminal membrane is increased by DNP. This evidence suggests the existence of a mediated transport system both in the luminal and the contraluminal cell membrane. Additionally, intracellular metabolism of hypoxanthine seems to regulate transepithelial permeation: increased hypoxanthine salvage by the phosphoribosyltransferase reduces the rate of secretion. However, the incorporation of hypoxanthine into the nucleotides is limited when the hypoxanthine is added to the luminal side of the epithelium, and the permeation rate in the absorptive direction is not markedly influenced by the rate of hypoxanthine salvage. These findings are a further example of the functional orientation of the jejunal epithelial cells with respect to enzymic activity and transepithelial transport properties.