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Blood flow in histamine‐ and allergen‐induced weal and flare responses, effects of an H1 antagonist, α‐adrenoceptor agonist and a topical glucocorticoid

Allergen has previously been shown to induce a continuous increase in local dermal blood flow after a prick test in allergic subjects, whereas histamine induced, initially, similar peak increases in blood flow of much shorter duration. Blood flow changes induced by histamine and allergen have now been evaluated (i) after pretreatment with a local corticosteroid cream, clobetasole‐17‐propionate; (ii) after oral administration of the H1‐antihistamine loratadine; and (iii) after oral pretreatment with the alpha‐adrenoceptor agonist pseudoephedrine. Blinded placebo‐controlled designs were used in the substudies. Laser doppler flowmetry was used for non‐invasive recording of changes in local blood flow intermittently for 24 h after the topical corticosteroid, 6 h for the substudies on loratadine and pseudoephedrine. The size of the immediate weal and flare reactions, as well as late phase reactions, were also determined. Prctreatment with clobetasole‐1 7‐propionate cream on the skin for 1 week prior to prick tests did not affect the blood flow response elicited by histamine or allergen, in either the initial part (up to 1 h) or the protracted 24 h determinations. The size of the weal and flare reactions decreased. Loratadine and pseudoephedrine did not reduce the initial allergen‐induced increase in blood flow, while lower blood flow compared with placebo pretreatment was noted for the protracted (1–6 h) determinations. Blood flow changes after histamine were unaffected. The histamine‐induced weal and flare was inhibited by loratadine more effectively than the corresponding allergen‐induced reaction. The weal and flare reactions after histamine and allergen were not changed after pseudoephedrine. The late dermal reaction expressed as the size of the erythema and/or induration present 6 h after allergen prick tests was reduced with pretreatment with loratadine as well as after seudoephedrine when compared with placebo. The results from these pharmacological experiments suggest that pretreatment with potent topical corticosteroids had no effect on changes in dermal blood flow, while H1‐antihistainines and α‐adrenoceptor agonists appear to have a direct effect on the vessels in a local allergic reaction.