ATP-gated ion channels (P2X receptors) are abundantly expressed in both neuronal and nonneuronal tissues, where they can serve as postsynaptic receptors. The response to ATP shows marked desensitization in some tissues but not others. Currents induced by ATP in Xenopus oocytes expressing cloned P2X ₁ (or P2X ₃ ) receptors had strong desensitization, whereas currents in cells expressing P2X ₂ receptors desensitized relatively little (90% vs. 14% decline of current in a 10-s application). In chimeric receptors, substitution into the P2X ₁ receptor of either one of two 34-residue segments from the P2X ₂ receptor removed the desensitization; these segments included the first or the second hydrophobic domain. In contrast, desensitization was introduced into the P2X ₂ receptor only by providing both these segments of the P2X ₁ (or P2X ₃ ) receptor. This suggests that desensitization requires interaction between the two hydrophobic domains of the receptor, and supports the view that these are membrane-spanning segments.

Mast cells play a significant role in the pathophysiology of many diverse diseases such as asthma and pulmonary fibrosis. Ca²⁺ influx is essential for mast cell degranulation and release of proinflammatory mediators, while Mg²⁺ plays an important role in cellular homeostasis. The channels supporting divalent cation influx in human mast cells have not been identified, but candidate channels include the transient receptor potential melastatin (TRPM) family. In this study, we have investigated TRPM7 expression and function in primary human lung mast cells (HLMCs) and in the human mast cell lines LAD2 and HMC-1, using RT-PCR, patch clamp electrophysiology, and RNA interference. Whole cell voltage-clamp recordings revealed a nonselective cation current that activated spontaneously following loss of intracellular Mg²⁺. The current had a nonlinear current-voltage relationship with the characteristic steep outward rectification associated with TRPM7 channels. Reducing external divalent concentration from 3 to 0.3 mM dramatically increased the size of the outward current, whereas the current was markedly inhibited by elevated intracellular Mg²⁺ (6 mM). Ion substitution experiments revealed cation selectivity and Ca²⁺ permeability. RT-PCR confirmed the presence of mRNA for TRPM7 in HLMC, LAD2, and HMC-1 cells. Adenoviral-mediated knockdown of TRPM7 in HLMC with short hairpin RNA and in HMC-1 with short interfering RNA markedly reduced TRPM7 currents and induced cell death, an effect that was not rescued by raising extracellular Mg²⁺. In summary, HLMC and human mast cell lines express the nonselective cation channel TRPM7 whose presence is essential for cell survival.