Histamine-induced cytosolic calcium mobilization in human bronchial smooth muscle cells

Histamine is a well-known mediator of bronchoconstriction. Despite the widespread use of histamine as a tool to study the bronchial smooth muscle function, the precise mechanism by which it causes calcium mobilization in bronchial smooth muscle cells remains unclear. Therefore, the current study aimed to investigate the mechanism of action of histamine in calcium mobilization in cultured human bronchial smooth muscle cells. A series of in vitro calcium imaging experiments have shown that histamine increases intracellular calcium levels in a concentration-dependent manner. The half maximum concentration of cytosolic Ca²⁺ peak was 3.00 ± 0.25 µM of histamine. Histamine was able to mobilize calcium from intracellular stores, even in the absence of extracellular calcium. These histamine-induced calcium elevations were completely blocked by the H₁ receptor antagonist chlorpheniramine (1 µM). Histamine-induced calcium elevation was also completely inhibited by the phospholipase C (PLC) inhibitor U73122 (1 µM) and inositol 1,4,5-trisphosphate (InsP₃) receptor inhibitor caffeine (20 mM). Cyanide p-(trifluoromethoxy)phenylhydrazone (1 µM) and oligomycin (1 µg/ml) effectively attenuated histamine-induced calcium release from intracellular stores. In the presence of histamine, cytosolic calcium elevation induced by reperfusion of 1.28 mM extracellular calcium after the depletion of stores was significantly inhibited by FCCP and oligomycin, unlike in the presence of thapsigargin. Based on the above results, we can conclude that histamine activates the intracellular PLC/InP₃ pathway through the H₁ receptor, which in turn activates the InP₃ receptor present in intracellular stores to mobilize calcium in human bronchial smooth muscle cells. In addition, the mitochondria appear to be involved in the release of calcium from intracellular stores. These results provide insights into the mechanisms underlying histamine-induced calcium mobilization for bronchoconstriction under pathophysiological conditions.