Plasma membrane hyperpolarization by cyanide in chromaffin cells: role of potassium channels

Abstract

Exposure of rat pheochromocytoma (PC12) cells to cyanide produces elevation of cytosolic calcium, impaired Na⁺−H⁺ exchange, membrane lipid peroxidation and release of neurotransmitters. Since these observations suggested cyanide alters plasma membrane function, the present study examined the effect of NaCN on the membrane potential of undifferentiated PC12 cells in suspension. In PC12 cells loaded with the voltage sensitive fluorescent dye, bis-oxonol, cyanide (2.5–10 mM) elicited an immediate (within seconds), concentration related decrease in fluorescence, indicating hyperpolarization of the plasma membrane. Increasing extracellular K⁺ concentration to 20 mM blocked the effect of cyanide (5 mM), suggesting cyanide increased K⁺ efflux. Pretreatment with quinine blocked the cyanide-induced hyperpolarization, whereas glyburide had little effect, showing the hyperpolarization produced by cyanide was due to activation of Ca²⁺ sensitive K⁺ channels. Removal of Ca²⁺ from the media did not influence cyanide-induced hyperpolarization. However, buffering intracellular Ca²⁺ by loading cells with the Ca²⁺ chelators, Quin II or BAPTA, abolished the cyanide effect, showing cytosolic Ca²⁺ is a key factor. These findings suggest that cyanide mobilizes Ca²⁺ from intracellular stores which leads to hyperpolarization via the activation of Ca²⁺ sensitive K⁺ channels.