Intracellular activities of sodium and potassium.

The intracellular contents of Na+ and K+ appear to modulate a variety of cellular and tissue functions, including transepithelial transport. However these total ionic contents appear to consist of heterogeneous populations of Na+ and K+. No more than a few percent of intracellular Na+ and K+ appear bound. Thus, the heterogeneity of the intracellular ionic contents must reflect subcellular compartmentalization. Recent technical advances in the preparation and use of ion-selective microelectrodes have permitted progress in defining the possible sites of such intracellular compartmentalization. Furthermore, intracellular recording with microelectrodes and micropipettes now provides a technique with which to directly monitor the chemical activities of Na+ and K+ within the cytoplasm as a function of the state of transepithelial transport. This approach has already provided information necessitating a reexamination of certain classical concepts of transport physiology.