EFFECTS OF IONIC-STRENGTH ON THE REGULATION OF NA H EXCHANGE AND K-CLCOTRANSPORT IN DOG RED-BLOOD-CELLS/

Dog red cell membranes contain two distinct volume-sensitive transport ers: swelling-activated K-CI cotransport and shrinkage-activated Na/H exchange. Cells were prepared with intracellular salt concentration an d weight percentage of cell water (%cw) varied independently by transi ent permeabilization of the cell membrane to cations. The dependence o f transporter-mediated Na and K influxes upon %cw and upon extracellul ar salt concentration (c(ext)) was measured in cells so prepared. It w as found that the critical value of %cw at which transporters are acti vated, called the set point, is similar for the two transporters, and that the set points for the two transporters decrease similarly with i ncreasing extracellular salt concentration. These findings suggest a c ommon mechanism of regulation of these two transporters. Cellular Na, K, and Cl concentrations were measured as functions of %cw and c(ext). Using these data together with data from the literature for other sol ute concentrations, empirical expressions were developed to describe t he dependence of the intracellular concentrations of all significant s mall molecule electrolytes, and therefore the intracellular ionic stre ngth, upon %cw and c(ext). A mechanistic model for the dependence of t he set point of an individual transporter upon intracellular ionic str ength is proposed. According to this model, the set point represents a critical extent of association between the transporter and a postulat ed soluble regulatory protein, called regulator. Model functions are p resented for the calculation of the thermodynamic activity of regulato r, and hence extent of regulator-transporter association, as a functio n of total intracellular protein concentration (or %cw) and ionic stre ngth. The experimentally observed dependence of set point %cw on c(ext ) are simulated using these functions and the empirical expressions de scribed above, together with reasonable but not uniquely determined va lues of model parameters.