Background Mast cell activation has been assumed to play a role in dermal neurogenic inflammation: C fibre-derived neuropeptides activating mast cells and releasing histamine, which in turn would activate C fibres. Objective To test this hypothesis mast cell tryptase (MCT) was measured inside the axon reflex flare area. Axon reflexes were elicited by histamine or compound 48/80, a polyanionic mast cell-degranulating substance. The time course of plasma extravasation and release of histamine and MCT from dermal mast cells in neurogenic inflammation was measured in vivo by intradermal microdialysis in humans. Methods Single hollow plasmapheresis fibres (pore cutoff size: 3000 kDa) were inserted intracutaneously at the volar forearm and perfused with Ringer’s solution (4 μL/min) with one microdialysis fibre located at the planned stimulation site and a second inside the axon reflex area. Neurogenic inflammation was induced by intraprobe delivery of either histamine or the mast cell-degranulating agent compound 48/80. Mediator release was measured at the stimulation sites and inside the arising axon reflex flare area. Results Mast cell degranulation induced marked plasma protein extravasation (PPE 0.25 ± 0.04–1.31 ± 0.6 mg/mL; pre- and post-stimulation, mean ± sem, n = 7) and release of histamine (2.0 ± 0.9–38.7 ± 1.4 ng/mL) and MCT (9.84 ± 2.4–92.2 ± 21.6 ng/mL). Interestingly, in addition to increasing PPE (0.33 ± 0.11–1.85 ± 0.9 mg/mL), histamine also induced a slight but significant increase in MCT (11.3 ± 3.0–12.4 ± 2.3 ng/mL). No evidence for mast cell activation was observed inside the axon reflex areas, where PPE (0.34 ± 0.03–0.25 ± 0.02 mg/mL), histamine (1.64 ± 0.5–1.46 ± 0.4 ng/mL) and MCT concentration (11.6 ± 3.1–7.6 ± 1.7 ng/mL) gradually decreased. Conclusion It is concluded that dermal neurogenic inflammation does not degranulate mast cells.
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