Effect of nordihydroguaiaretic acid on intracellular Ca²⁺ concentrations in hepatocytes

The effect of nordihydroguaiaretic acid (NDGA) on Ca²⁺ signaling in human hepatoma cells (HA22/VGH) has been investigated. NDGA (5-50 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 10-50 μM NDGA induced [Ca²⁺]_i signals by 45±5%. Consistently, the 50 μM NDGA-induced [Ca²⁺]_i increase in Ca²⁺-containing medium was reduced by 41±2% by 10 μM of La³⁺, nifedipine or verapamil. In Ca²⁺-free medium, pretreatment with 20 μM NDGA for 6 min abolished the [Ca²⁺]_i increase induced by the endoplasmic reticulum Ca²⁺ pump inhibitor thapsigargin (1 μM). Conversely, 20 μM NDGA failed to increase [Ca²⁺]_i after 1 μM thapsigargin had depleted the endoplasmic reticulum Ca²⁺ store. Inhibition of phospholipase C with 2 μM U73122 had little effect on 20 μM NDGA-induced Ca²⁺ release. Several other lipoxygenase inhibitors had no effect on basal [Ca²⁺]_i. Together, the data suggest that NDGA increased [Ca²⁺]_i in hepatocytes in a lipoxygenase-independent manner, by releasing Ca²⁺ from the endoplasmic reticulum and causing Ca²⁺ influx.