Histamine-induced increases in intracellular free ca ²⁺ levels in hepatoma cells

The effect of histamine on intracellular free Ca ²⁺ levels ([Ca ²⁺ ] _i ) in HA22/VGH human hepatoma cells were evaluated using fura-2 as a fluorescent Ca ²⁺ dye. Histamine (0.2-5 μM) increased [Ca ²⁺ ] _i in a concentration-dependent manner with an EC ₅₀ value of about 1 μM. The [Ca ²⁺ ] _i response comprised an initial rise, a slow decay, and a sustained phase. Extracellular Ca ²⁺ removal inhibited 50% of the [Ca ²⁺ ] _i signal. In Ca ²⁺ -free medium, after cells were treated with 1 μM thapsigargin (an endoplasmic reticulum Ca ²⁺ pump inhibitor), 5 μM histamine failed to increase [Ca ²⁺ ] _i . After pretreatment with 5 ↑ M histamine in Ca ²⁺ -free medium for 4 min, addition of 3 mM Ca ²⁺ induced a [Ca ²⁺ ] _i increase of a magnitude 7-fold greater than control. Histamine (5 μM)-induced intracellular Ca ²⁺ release was abolished by inhibiting phospholipase C with 2 μM 1-(6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2, 5-dione (U73122), and by 5 μM pyrilamine but was not altered by 50 μM cimetidine. Together, this study shows that histamine induced [Ca ²⁺ ] _i increases in human hepatoma cells by stimulating H1, but not H2, histamine receptors. The [Ca ²⁺ ] _i signal was caused by Ca ²⁺ release from thapsigargin-sensitive endoplasmic reticulum in an inositol 1,4,5-trisphosphate-dependent manner, accompanied by Ca ²⁺ entry.