Extracellular Charge Adsorption Influences Intracellular Electrochemical Homeostasis in Amphibian Skeletal Muscle

The membrane potential measured by intracellular electrodes, Em, is the sum of the transmembrane potential difference (E1) between inner and outer cell membrane surfaces and a smaller potential difference (E2) between a volume containing fixed charges on or near the outer membrane surface and the bulk extracellular space. This study investigates the influence of E2 upon transmembrane ion fluxes, and hence cellular electrochemical homeostasis, using an integrative approach that combines computational and experimental methods. First, analytic equations were developed to calculate the influence of charges constrained within a three-dimensional glycocalyceal matrix enveloping the cell membrane outer surface upon local electrical potentials and ion concentrations. Electron microscopy confirmed predictions of these equations that extracellular charge adsorption influences glycocalyceal volume. Second, the novel analytic glycocalyx formulation was incorporated into the charge-difference cellular model of Fraser and Huang to simulate the influence of extracellular fixed charges upon intracellular ionic homeostasis. Experimental measurements of Em supported the resulting predictions that an increased magnitude of extracellular fixed charge increases net transmembrane ionic leak currents, resulting in either a compensatory increase in Na+/K+-ATPase activity, or, in cells with reduced Na+/K+-ATPase activity, a partial dissipation of transmembrane ionic gradients and depolarization of Em.