Mechanism of monensin-induced hyperpolarization of neuroblastoma-glioma hybrid NG108-15.

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Abstract
Addition of the ionophore monensin to mouse neuroblastoma-rat glioma hybrid NG108-15 cells leads to a 20-30 mV increase in the electrical potential across the plasma membrane as shown by direct intracellular recording techniques and by distribution studies with the lipophilic cation [3H]tetraphenylphosphonium+ (TPP+). The effect is not observed with cells suspended in the high K+ medium, is dependent upon the presence of Na+ externally, and the concentration of monensin that induces half-maximal stimulation of TPP+ accumulation is .apprx. 1 .mu.M. The ionophore causes rapid influx of Na+, a transient increase in intracellular pH and a decrease in extracellular pH, all of which are consistent with the known ability of monensin to catalyze the transmembrane exchange of H+ for Na+. Although ouabain has no immediate effect on the membrane potential, the cardiac glycoside completely blocks the increase in TPP+ accumulation observed in the presence of monensin. The hyperpolarizing effect of monensin is mediated apparently by an increase in intracellular Na+ that acts to stimulate the electrogenic activity of the Na+, K+-ATPase. Because monensin stimulated TPP+ accumulation in a number of other cultured cell lines in addition to NG108-15, the techniques described may be of general use for studying the the Na+,K+ pump and its regulation in situ.