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The electro physiology of rabbit descending colon 1. instantaneous trans epithelial current voltage relations and the current voltage relations of the sodium entry mechanism

, : The electro physiology of rabbit descending colon 1. instantaneous trans epithelial current voltage relations and the current voltage relations of the sodium entry mechanism. Journal of Membrane Biology 66(1): 41-54

A method is described for determining the instantaneous transepithelial current-voltage (I-V) relations across rabbit descending colon and deriving the I-V relations of the amiloride-sensitive Na-entry step across the apical membrane. The latter conforms closely to the predictions of the Goldman-Hodgkin-Katz constant-field flux equation over a wide range of values of the transapical electrical potential difference (-120 to +50 mV), suggesting that Na entry is the result of simple electrodiffusion through homogeneous pores or channels. The permeability of the apical membrane to Na averaged 0.012 cm/h and the intracellular Na activity averaged 10 mM. The rate of Na entry across the apical membrane varied, spontaneously, over a 4-fold range; this variation is entirely attributable to parallel variations in the partial conductance of the apical membrane to Na with no change in the driving force for this movement. Bathing the serosal surface of the tissue with a high-K solution abolishes the electrical potential difference across the basolateral membrane and markedly reduces the resistance of that barrier. Under these conditions, the I-V relations of the amiloride-sensitive Na-entry step across the apical membrane also conform closely to the predictions of the constant-field flux equation. The significance of the point at which the transepithelial I-V relations in the absence and presence of amiloride intersect (ENa) and the origin of the bends in these I-V relations at or around this point are discussed. The point of intersection is simply that value of the transepithelial electrical potential difference at which Na entry is abolished and has no direct bearing on the energetics of the basolateral pump. The bend in the I-V relations appears to be due to an increase in the conductance of a pathway in the apical membrane that parallels the Na-entry pathway, as well as an increase in the conductance of the paracellular pathway; thus, this bend does not appear to be directly related to changes in the active Na transport pathway.

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