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

Summary The current-voltage (I–V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane ofNecturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, ^mc , and that across the entire epithelium, ^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 ^mc over the range –50 mV < ^mc <E _SNa ^m whereE _SNa ^m is the zero current or reversal potential. Over the range of values of ^mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so thatI _SNa ^m =g _SNa ^m (E _SNa ^m – ^mc ) whereg _SNa ^m is the contribution of this mechanism to the conductance of the apical membrane and is near constant. In several instancesI _SNa ^m saturated at large hyperpolarizing or depolarizing values of ^mc .The values ofE _SNa ^m determined in the presence of 1, 5, and 15mm 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 observedI–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.