Previous results have shown a dose-dependent inhibition of the phytohaemagglutinin-elicited chemiluminescent response by histamine on human peripheral blood mononuclear cells (PBMs). The aim of the present experiments was to investigate the receptor specificity of this histamine action. The order of effectiveness of different histaminergic agonists was 4-methylhistamine > histamine = impromidine > dimaprit a ̊ 2-methylhistamine > 2-pyridylethylamine. Cimetidine inhibited and mepyramine enhanced this effect of histaminergic agonists. The results are consistent with the view that histamine inhibits the chemiluminescent reaction of PBMs via H2 receptors. Histamine in low doses (10-8 to 10-10 M) stimulated the chemiluminescent reaction. Cimetidine enhanced the chemiluminescence-facilitatory action of histamine and unmasked that of 2-pyridylethylamine. It is concluded that histamine is a possible humoral modulator of PBM activity: it is inhibitory via H2 receptors.
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