Effect of toluene diisocyanate on homeostasis of intracellular-free calcium in human neuroblastoma SH-SY5Y cells.

The mechanisms of TDI (2,4-toluene diisocyanate)-induced occupational asthma are not fully established. Previous studies have indicated that TDI induces non-specific bronchial hyperreactivity to methacholine and induces contraction of smooth muscle tissue by activating 'capsaicin-sensitive' nerves resulting asthma. Cytosolic-free calcium ion concentrations ([Ca(2+)](c)) are elevated when either capsaicin acts at vanilloid receptors, or methacholine at muscarinic receptors. This study therefore investigated the effects of TDI on Ca(2+) mobilization in human neuroblastoma SH-SY5Y cells. TDI was found to elevate [Ca(2+)](c) by releasing Ca(2+) from the intracellular stores and extracellular Ca(2+) influx. 500 microM TDI induced a net [Ca(2+)](c) increase of 112+/-8 and 78+/-6 nM in the presence and absence of extracellular Ca(2+), respectively. In Ca(2+)-free buffer, TDI induced Ca(2+) release from internal stores to reduce their Ca(2+) content and this reduction was evidenced by a suppression occurring on the [Ca(2+)](c) rise induced by thapsigargin, ionomycin, and methacholine after TDI incubation. In the presence of extracellular Ca(2+), simultaneous exposure to TDI and methacholine led a higher level of [Ca(2+)](c) compared to single methacholine stimulation, that might explain that TDI induces bronchial hyperreactivity to methacholine. We conclude that TDI is capable of interfering the [Ca(2+)](c) homeostasis including releasing Ca(2+) from internal stores and inducing extracellular Ca(2+) influx. The interaction of this novel character and bronchial hyperreactivity need further investigation.