Zn²⁺ increases resting cytosolic Ca²⁺ levels and abolishes capacitative Ca²⁺ entry induced by ATP in MDCK cells

The effect of Zn²⁺ on Ca²⁺ signaling in Madin Darby canine kidney (MDCK) cells was investigated by measuring the changes in the fluorescence of the Ca²⁺sensitive dye fura-2. Zn²⁺ significantly increased cytoplasmic free Ca²⁺ levels ([Ca²⁺](i)) at concentrations of 2-100 μM. The maximum response was obtained at concentrations of 25-100 μM. The [Ca²⁺](i) rise induced by 100 μm Zn²⁺consisted of a gradual rise and a plateau phase, and was primarily mediated by La³⁺-sensitive extracellular Ca²⁺ influx because the [Ca²⁺](i) rise was abolished by pretreatment with 100 μM La³⁺ or removal of extracellular Ca²⁺, and that Zn²⁺ induced Mn²⁺ quench of fura-2 fluorescence at 360 nm excitation wavelength which was prevented by pretreatment with 100 μM La³⁺. Pretreatment with 100 μM Zn²⁺ for 220 s did not reduce the [Ca²⁺](i) rise induced by the endoplasmic reticulum (ER) Ca²⁺ pump inhibitor, thapsigargin, suggesting that Ca²⁺ release from the ER played a minor role in the Zn²⁺-induced [Ca²⁺](i) rise. Zn²⁺ (100 μM) nearly abolished the capacitative Ca²⁺ entry induced by ATP (100 μM). We also investigated the effect of Zn²⁺ pretreatment on the [Ca²⁺](i) rise induced by ATP. Zn²⁺ (100 μM) affected ATP-induced [Ca²⁺](i) rise by abolishing capacitative Ca²⁺ entry and increasing [Ca²⁺] on its own without altering Ca²⁺ release from intracellular sources. The effect of Zn²⁺ on [Ca²⁺](i) was dissociated from changes in membrane potential.