Histamine metabolism in the nasal tissue of human and nasal hypersensitive guinea-pig

Histamine (HA) is the most important mediator of nasal allergy and nasal hypersensitivity. To investigate HA metabolism, HA content and activities of its synthetic enzyme, histidine decarboxylase (HDC) and degrading enzymes, histamine-N-methyltransferase (HMT) and diamine oxidase (DAO) in nasal mucosa of human and toluene diisocyanate (TDI) sensitized guinea-pigs were measured. In human nasal mucosa and nasal polyps, HA content and HDC activity were 80-200 nmol/g tissue, 20-30 fmol/min/mg protein respectively. Among two degrading enzymes, HMT activity was 20-200 times higher than that of DAO. In the nasal mucosa of guinea-pigs, HA content was significantly increased by TDI sensitization, and was decreased immediately after TDI provocation. In 24 hours after provocation, HA content recovered to 80% of pre-provocation level. HDC activity increased by TDI sensitization significantly. Though HMT activity increased slightly by TDI sensitization and provocation, DAO activity was unchanged. The data suggest that, increase in turnover rate of HA is present in allergic nasal mucosa.