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 (a_K) and the K⁺ equilibrium potential (E_K) are measured using ion-sensitive microelectrodes. The mean value of a_K 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 E_K obtained from the Nernst equation are −81 ± 2 mV and −40 ± 2 mV, respectively. The reversal potentials (E_R) 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 E_R 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 E_K 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.