Passive cation permeability of turtle colon: Evidence for a negative interaction between intracellular sodium and apical sodium permeability

The role of intracellular sodium in the regulation of apical sodium permeability was investigated in an electrically “tight” epithelium, the turtle colon. In the presence of low mucosal sodium (3 mM) and serosal ouabain, an inhibitor of the basolateral sodium pump, the apical membrane retained a substantial amiloride-sensitive, sodium conductance and the basolateral membrane exhibited a barium-sensitive potassium conductance in parallel with a significant sodium (and lithium) conductance. In the presence of a high mucosal sodium concentration (114 mM), however, inhibition of active sodium absorption by ouabain led to a disappearance of the amiloride-sensitive, transepithelial conductance that was due, at least in part, to a virtual abolition of the apical sodium permeability. Two lines of evidence indicate that this permeability decrease was dependent upon an increase in intracellular sodium content. First, raising the mucosal sodium concentration from 3–114 mM in the presence of ouabain reversibly inhibited the amiloride-sensitive conductance. The time course of the decline in conductance paralleled the apparent intracellular accumulation of sodium in exchange for potassium, which was monitored as a transient deflection in the amiloride-sensitive, short-circuit current. Second, the inhibitory effect of mucosal sodium-addition was markedly attenuated by serosal barium, which prevented the accumulation of sodium by blocking the electrically coupled, basolateral potassium exit. These results support the notion of a “negative feedback” effect of intracellular sodium on the apical sodium permeability.