Intestinal sugar transport: does the Na+ gradient provide all the energy?

It has not yet been established that the energy released from the Na+ gradient during Na+-sugar cotransport by small intestine is sufficient, by itself, to drive uphill sugar transport under conditions in which other sources of energy are available and in which sugar is being vigorously transported. This work attempts to test the thermodynamic sufficiency of the Na+ mechanism under such conditions using hamster jejunum in vitro. From data on tissue uptake of D-galactose and unstirred layer considerations, the concentration ratio for galactose across the apical membrane during mucosal-to-serosal galactose transport was estimated to be at least 50 when concentration in the bulk mucosal solution was 2.2 mM. This estimate and reasonable assumptions regarding intracellular Na+ activity and apical membrane potential were used to calculate the minimum physiological coupling ratio (i.e., the ratio of coupled net fluxes of Na+ and galactose) required if all the energy necessary for galactose transport is transferred from Na+. The minimum physiological coupling ratio required was roughly 1.4. An estimate based on the ratio of transepithelial chord permeabilities to galactose, instead of the apical concentration ratio, would be higher. The measured physiological coupling ratio (0.4) was considerably less than that required. For reasons discussed, this result should be interpreted with caution. It can be concluded that the issue of energetic adequacy is not yet settled.