Histamine stimulates normal human melanocytes in vitro: one of the possible inducers of hyperpigmentation in urticaria pigmentosa.

Isolated normal human melanocytes became enlarged and more dendritic in association with an increase in the activity of tyrosinase and the amount of b-locus protein when they were cultured with 0.1-10 microM histamine in vitro. However, histamine did not exert a proliferative effect on them. The stimulatory effect of histamine was observable even 6 h after starting the treatment. This stimulation seems not to be pharmacologically mediated through histamine receptors, because it was inhibited neither by pyrilamine, a histamine H-1 antagonist, nor by cimetidine, a H-2 antagonist. Imidazole derivatives that are rapidly metabolized from histamine in vivo and in vitro also stimulated the melanocytes. We propose that high concentrations of histamine and its imidazole metabolites continuously produced in the lesions of urticaria pigmentosa are probable causative factors of its characteristic skin pigmentation.