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, Ψmc, and that across the entire epithelium, Ψms, 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 mSNa , is blocked by the addition of phloridzin to the mucosal solution.I mSNa was found to be strongly dependent upon Ψmc over the range −50 mV < Ψmc <E mSNa whereE mSNa is the “zero current” or “reversal” potential. Over the range of values of Ψmc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so thatI mSNa =g mSNa (E mSNa − Ψmc) whereg mSNa is the contribution of this mechanism to the conductance of the apical membrane and is “near constant”. In several instancesI mSNa “saturated” at large hyperpolarizing or depolarizing values of Ψmc.
The values ofE mSNa 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.
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Lapointe, JY., Hudson, R.L. & Schultz, S.G. Current-voltage relations of sodium-coupled sugar transport across the apical membrane ofNecturus small intestine. J. Membrain Biol. 93, 205–219 (1986). https://doi.org/10.1007/BF01871175
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DOI: https://doi.org/10.1007/BF01871175