RESPONSES OF HUMAN SKIN BLOOD VESSELS TO SYNTHETIC HISTAMINE ANALOGUES

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Abstract
The vascular responses of human skin to two synthetic analogues of histamine, 2‐methyl histamine (an H1‐receptor agonist) and 4‐methyl histamine (an H2‐receptor agonist) have been studied in vivo. Both compounds evoked dose‐related erythema, 2‐methyl histamine but not 4‐methyl histamine causing erythema mediated by an axon reflex, thus suggesting that the axon reflex and direct vasodilator action of histamine are due to H1 and H2 actions respectively. The H1‐receptor antagonist chlorpheniramine inhibited erythema due to 2‐methyl histamine. Cimetidine, an H2‐receptor antagonist, had no effect on the reaction to 2‐methyl histamine. In contrast, the erythema reaction to 4‐methyl histamine was suppressed by both cimetidine and chlorpheniramine. Although both histamine analogues caused wealing, this was not dose‐related within the dose range used, and neither chlorpheniramine nor cimetidine caused detectable suppression of wealing responses to either histamine analogue. These results lend further support to the view that human skin blood vessels possess H2 as well as H1 receptors.