Transient receptor potential melastatin subfamily 3 (TRPM3) ion channels play a role in calcium (Ca²⁺ ) cell signalling. Reduced TRPM3 protein expression has been identified in chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) patients. However, the significance of TRPM3 and association with intracellular Ca²⁺ mobilization has yet to be determined. Fifteen CFS/ME patients (mean age 48·82 ± 9·83 years) and 25 healthy controls (mean age 39·2 ± 12·12 years) were examined. Isolated natural killer (NK) cells were labelled with fluorescent antibodies to determine TRPM3, CD107a and CD69 receptors on CD56^dim CD16⁺ NK cells and CD56^bright CD16^dim/- NK cells. Ca²⁺ flux and NK cytotoxicity activity was measured under various stimulants, including pregnenolone sulphate (PregS), thapsigargin (TG), 2-aminoethoxydiphenyl borate (2APB) and ionomycin. Unstimulated CD56^bright CD16^dim/- NK cells showed significantly reduced TRPM3 receptors in CFS/ME compared with healthy controls (HC). Ca²⁺ flux showed no significant difference between groups. Moreover, PregS-stimulated CD56^bright CD16^dim/- NK cells showed a significant increase in Ca²⁺ flux in CFS/ME patients compared with HC. By comparison, unstimulated CD56^dim CD16⁺ NK cells showed no significant difference in both Ca²⁺ flux and TRPM3 expression. PregS-stimulated CD56^dim CD16⁺ NK cells increased TRPM3 expression significantly in CFS/ME, but this was not associated with a significant increase in Ca²⁺ flux. Furthermore, TG-stimulated CD56^dim CD16⁺ NK cells increased K562 cell lysis prior to PregS stimulation in CFS/ME patients compared with HC. Differential expression of TRPM3 and Ca²⁺ flux between NK cell subtypes may provide evidence for their role in the pathomechanism involving NK cell cytotoxicity activity in CFS/ME.

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