Intracellular sodium activity and transcellular sodium transport in gallbladder.

As stated in the introduction, the purpose of this report has been to illustrate how the measurement of steady-state intracellular ionic activities, a technique that has proved to be of great importance in studying the energetics of transmembrane ionic transfer processes, can, with appropriate assumptions, be used to obtain information concerning the kinetics of these processes. Specifically, our analysis has focussed on transcellular Na+ transport in Necturus gallbladder and has shown that, given the steady-state values of Em, a1Na, and apical Na+ conductance for a particular set of conditions, it is possible to obtain estimates of net baso-lateral Na+ efflux (Na+ pump rate), net (and/or unidirectional) diffusive apical Na+ influx, and net coupled NaCl influx. It should be emphasized that the analysis outlined above is a preliminary essay in this direction. We present it here in the hope that, wit appropriate refinements, it may prove useful in unraveling the mechanisms by which drugs, hormones and other specific agents affect membrane function in epithelial and other systems.