Metabolism of [ 3H]ecdysone by isolated tissues of the female ixodid tick Amblyomma hebraeum (Ixodoidea; Ixodidae)

Abstract

Malpighian tubules, gut, ovaries and carcasses of the adult female tick Amblyomma hebraeum were incubated in vitro in the presence of 2 μM [ 3H]ecdysone. Organs and media were separately extracted after 6, 24 and 48 h incubations and the patterns of ecdysone metabolites were analyzed by HPLC. Esterase-susceptible apolar metabolites similar to the AP2 already described in the soft tick Ornithodoros moubata and thus presumably corresponding to the same conjugates (C-22 esters with fatty acids) were rapidly produced in all tissues investigated. They were mainly found within the organs but they were also released into the medium to some extent. By contrast, less apolar metabolites corresponding to the AP1 esters were mainly found in the media. Malpighian tubules and gut were the most active organs regarding the conversion of ecdysone into 20-hydroxyecdysone (20E). However, only low quantities of 20E were formed, reaching respectively 12.5% and 11.6% of the total metabolites after 48 h incubations. In the carcass and in the ovary the formation of 20E was only a minor pathway (1.7% and 3.1% of the total metabolites after 48 h). In ovaries we observed a massive conversion of ecdysone into 3-epiecdysone, which (as in insects) presumably proceeded through the intermediate formation of 3-dehydroecdysone. These two compounds were identified among the metabolites by CI/D mass spectrometry. The 3-epimer was released into the media, in contrast with the AP2 which were essentially stored within ovaries. Epimerization was also realized to some extent by carcasses, and again the epimer was released into the culture media. The different pathways are compared with those found in other tick species and in insects, and the significance of the various metabolites is discussed.