Studies on histamine metabolism in allergen-induced asthma

The excretion of histamine (Hi) and it metabolite methyhistamine (MeHi) was determined in separated fractions of urine up to 12h after standardized allergen provocations in 18 adult patients with defined extrinsic bronchial asthma. The main histamine metabolite, methylimidazoleacetic acid (MeImAA), was measured in six of the patients. After positive provocations (decrease in FEV1 greater than 20%) the excretion of Hi was significantly increased during 3h and that of MeHi during 4h after challenge. Negative provocations (decrease in FEV1 less than 20%) were not followed by any changes in the excretion of Hi and MeHi. MeImAA excretion increased in five out of six patients after positive provocation. It was calculated that the increased excretion of Hi and its metabolites after a positive provocation corresponded to a release of about 1 mg histamine in the body or about 1 microgram/g lung tissue if all histamine was liberated in the lung. Pretreatment with two anti-allergic drugs, disodium cromoglycate and ICI 74.917, giving significant allergen protection, resulted in a smaller increase of the excretion of both Hi and MeHi, indicating an inhibition of histamine release in vivo.