Electrophysiology of phagocytic membranes: intracellular K+ activity and K+ equilibrium potential in macrophage polykaryons

The role of K+ as current carrier during the slow membrane hyperpolarizations (SH) elicited by iontophoretic Ca2+ injections into macrophage polykaryons is studied. The intracellular K+ activity (ak) and the K+ equilibrium potential (Ek) are measured using ion-sensitive microelectrodes. The mean value of ak is 84 +/- 5 mM in a culture medium containing 5.3 mM K+, but increases to 100 +/- 8 mM when the extracellular K+ concentration is raised to 30.3 mM. Under the same conditions the values of Ek obtained from the Nernst equation are -81 +/- 2 mV and -40 +/- 2 mV, respectively. The reversal potentials (ER) of the SH are calculated from changes observed in transmembrane potential and input resistance, according to an equivalent model based only on passive ionic fluxes. The mean ER values obtained are -74 +/- 8 mV in the presence of low K+ concentration and -37 +/- 3 mV for the high K+ medium. These values are significantly smaller than the estimated Ek for the corresponding situations. Evidence for the existence of an electrogenic (Na+ + K+)-ATPase activity is also presented. The evidence indicates that an increase in the membrane potassium permeability can account for about 90% of the total permeability change occurring during the SH.