Current-voltage relations of sodium-coupled sugar transport across the apical membrane of Necturus small intestine

The current-voltage (I-V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane of Necturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, Ψ^mc, and that across the entire epithelium, Ψ^ms, when the latter was varied over the range ±200 mV, (i) under steady conditions in the presence of galactose and (ii) after the current across the apical membrane carried by the cotransporter, I_SNa^m, is blocked by the addition of phloridzin to the mucosal solution. I_SNa^m was found to be strongly dependent upon Ψ^mc over the range -50 mV < Ψ^mc <E_SNa^m where E_SNa^m is the "zero current" or "reversal" potential. Over the range of values of Ψ^mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so that I_SNa^m =g_SNa^m (E_SNa^m- Ψ^mc) where g_SNa^m is the contribution of this mechanism to the conductance of the apical membrane and is "near constant". In several instances I_SNa^m "saturated" at large hyperpolarizing or depolarizing values of Ψ^mc. The values of E_SNa^m determined in the presence of 1, 5, and 15 mm galactose strongly suggest that if the Na-galactose cotransporters are kinetically homogeneous, the stoichiometry of this coupled process is unity. Finally, the shapes of the observed I-V relations are consistent with the predictions of a simple kinetic model which conforms with current notions regarding the mechanico-kinetic properties of this cotransport process.