Does calcium couple the apical and basolateral membrane permeabilities in epithelia?

Tight epithelial cells actively transport sodium against steep electrochemical gradients. To maintain a constant internal ionic content and volume, they must continuously adjust the passive cation permeabilities of their membranes as the rate of transport varies. There is evidence suggesting that changes in cell calcium may accomplish this task. An increase in cell calcium reduces the luminal sodium permeability and increases basolateral potassium permeability. There is basolateral sodium-calcium exchange through which changes in the rate of sodium transport, reflected in the cell sodium activity, are translated into changes in cell calcium. To demonstrate that cell calcium couples the permeability of the cell membrane requires obtaining measurements of cell calcium activity under physiologically relevant conditions, and, to date, there are no measurements during spontaneous changes in the rate of transport. However, there are measurements following ouabain inhibition of the pump indicating that the increase is sufficient to account for the reduction in luminal sodium permeability observed in intact tissues.