Histamine receptors in human skin: indirect evidence.

Use of differential effects of agonists and antagonists has provided an experimental basis for subdivision of histamine receptors into H1 and H2. This has enabled classification of the responses to histamine in a wide range of organs and tissues. In human skin the vascular and sensory effects of histamine have been studied, and the influence of histamine on epidermal cell growth has been evaluated in vitro. The direct vasodilator and vascular permeability actions of histamine appear to involve both H1 and H2 receptors. However, the axon reflex flare due to histamine appears to be predominantly an H1 effect. Histamine itch involves H1 receptors. These findings have prompted clinical evaluation of combined therapy with H1 and H2 antagonists in patients with urticaria. Initial results have been encouraging in patients with factitious urticaria. Histamine exerts an inhibitory action on epidermopoiesis in vitro in a number of mammalian epidermal cell outgrowth systems including human explants. The pathophysiological relevance of this effect, which involves epidermal H2 receptors and which may be cyclic AMP dependent, is unknown.