Cation selectivity of the apical membrane of the turtle colon: Sodium entry in the presence of lithium

Exposure of the apical surface of the isolated turtle colon to Li produced a marked transient in short-circuit current (I_sc) and total tissue conductance (G_T) which was abolished by amiloride but was unaffected by ouabain or by removing Na or Cl from the mucosal bathing solution. Despite marked changes in I_sc, Na uptake across the apical membrane was a linear function of time during exposure to Li-containing solutions, and except at very high Li concentrations, the initial rate of Na uptake, J_i^Na, was identical to its pre-Li value. In the presence of Li, however, J_i^Na was significantly less than the total I_sc. The apparent "transference number" for Na in the apical membranes was a function of the Li: Na concentration ratio in the mucosal bathing solution. These results suggest that Li can carry substantial amounts of current through amiloride-sensitive channels in the apical membrane of the colon without having any effect on the rate coefficient for Na entry. This behavior is not consistent with "competition" of Na and Li for a membrane "carrier" but rather suggests that the Na entry mechanism may be a population of pores or channels through which Na and Li may pass with negligible interaction.