Effect of nordihydroguaiaretic acid on intracellular Ca²⁺ concentrations in C6 glioma cells

The effect of nordihydroguaiaretic acid (NDGA) on Ca²⁺ signaling in C6 glioma cells has been investigated. NDGA (5-100 μM) increased [Ca²⁺]_i concentration-dependently. The [Ca²⁺]_i increase comprised an initial rise and an elevated phase over a time period of 4 min. Removal of extracellular Ca²⁺ reduced NDGA-induced [Ca²⁺]_i signals by 52 ± 2%. After incubation of cells with NDGA in Ca²⁺-free medium for 4 min, addition of 3 mM CaCl₂ induced a concentration-dependent increase in [Ca²⁺]_i. NDGA (100 μM)-induced [Ca²⁺]_i increases in Ca²⁺-containing medium was not changed by pretreatment with 10 μM nifedipine or verapamil. In Ca²⁺-free medium, pretreatment with the endoplasmic reticulum Ca²⁺ pump inhibitor thapsigargin (1 μM) abolished 100 μM NDGA-induced [Ca²⁺]_i increases. Inhibition of phospholipase C with 2 μM U73122 had little effect on 100 μM NDGA-induced Ca²⁺ release. Several other lipoxygenase inhibitors had no effect on basal [Ca²⁺]_i. Collectively, the results suggest that NDGA increased [Ca²⁺]_i in glioma cells in a lipoxygenase-independent manner, by releasing Ca²⁺ from the endoplasmic reticulum in a manner independent of phospholipase C activity and by causing Ca²⁺ influx.