Galactose transport across the serosal border of rabbit ileum and its role in intracellular accumulation.

Unidirectional fluxes of D-galactose across the brush and serosal border of rabbit ileum were determined using the method described previously (Naftalin, R. J. and Curran, P.F. (1974) J. Membrane Biol. 16, 257-278). With ringer [Na] equals 75 meguiv., the Km for galactose influx across the brush-border is 5mM, with 0.1 mM ouabain present K-m equals 50 mM, the V (2.0 munol - CM-2-H-1) remains unaltered. The Michaelis parameters for galactose influx across the serosal border are K-m equals 59 plus or minus 9 mM and V equals 4.7 plus or minus 0.24 mumol-cm-2-h-1 and for efflux K-m equals 85 plus or minus 10 mM and V equals 6.8 plus or minus 0.7 mumol-CM-2-H-1. 2. 2-Deoxy-D-glucose and methyl beta-D-glucopyranoside inhibit galactose entry exclusively at the serosal and mucosal borders respectively, while 3-O-methyl-D-glucose inhibits galactose influx at both borders. 0.1 mM ouabain increases the K1 of 3-O-methylglucose for the serosal transport system (100 mM) is unaffected by ouabain. Inhibition of mucosal galactose transport by ouabain or by competition with other sugars results in a reciprocal increase in exit permeability and decrease in entry permeability. Inhibition of serosal galactose transport results in inhibition of both the entry and exit permeability, entry is more affected. 3. There is a small degree of permeability asymetry at the serosal border to galactose which is reduced by ouabain or removel of Na+ from the Ringer. Uptake of 14C-labelled galactose from the serosal solution into the tissue is also inhibited by addition of ouabain or Na+ removal. It is therefore considered that there is a weak active transport system for galactose at the serosal border. 4. Net transepithelial galactose flux is sufficiently high and serosal permeability to galactose sufficiently low to be consistent with the view that galactose is concentrated within the tissue fluid, after conviction (Naftalin, R.J. and Holman, G.D. (1974) Biochim. Biophys. Acta., 373, 453-470) across the mucosal border because it is reflected at the serosal boundary.