Prevention of monensin-induced hyperpolarization in NG108-15 cells

The NG 108-15 (neuroblastoma X glioma hybrid) cell line was used as an in vitro neuronal model to evaluate potential antagonists of the Na+-selective carboxylic ionophore monensin. Changes in membrane electrical characteristics induced by monensin with and without the simultaneous administration of antagonists were measured using intracellular microelectrode techniques. Bath application of monensin (3 microM) produced a hyperpolarization of approximately = 35 mV. Monensin also altered the generation of action potentials in response to electrical stimulation in 14 of 24 (58%) exposed cells, as evident in a partial or complete loss of action potentials or in an alteration of action potential waveform. The antagonists used were Na+-K+ pump inhibitor ouabain (1-3 microM), the Ca2+dependent K+ channel blocker quinine (3-30 microM) or drugs known to influence Ca2+ signaling in cells, i.e., trifluoperazine (3-10 microM), verapamil (1-10 microM) or chlorpromazine (3-30 microM). On a molar basis, ouabain was the most and trifluoperazine the least effective of the antagonists. Quinine, verapamil and chlorpromazine all prevented the development of the hyperpolarization in an approximate concentration-dependent manner. However, none of these drugs was able to block the effects of monensin on action potentials. Indeed, high concentrations of the antagonists that were most effective in preventing the hyperpolarization accentuated impairments in action potential generation and also reduced input resistance in many cells. Thus, none of these antagonists appears suitable for transition to in vivo antidotal protection studies.