Physiological screening for target site insensitivity and localization of Na(+)/K(+)-ATPase in cardenolide-adapted...

Cardenolides are toxic plant compounds which specifically inhibit Na(+)/K(+)-ATPase, an animal enzyme which is essential for many physiological processes, such as the generation of action potentials. Several adapted insects feeding on cardenolide-containing plants sequester these toxins for their own defence. Some of these insects were shown to possess Na(+)/K(+)-ATPases with a reduced sensitivity towards cardenolides (target site insensitivity). In the present study we screened five species of arctiid moths feeding on cardenolide-containing plants for target site insensitivity towards cardenolides using an in vitro enzyme assay. The derived dose response curves of the respective Na(+)/K(+)-ATPases were compared to the insensitive Na(+)/K(+)-ATPase of the monarch butterfly (Danaus plexippus). Na(+)/K(+)-ATPases of all arctiid species tested were highly sensitive to ouabain, a water-soluble cardenolide which is most widely used in laboratory studies. Nevertheless, we detected substantial amounts of cardenolides in the haemolymph of two of the arctiid species. In caterpillars of the sequestering arctiid Empyreuma pugione and of D. plexippus we localized Na(+)/K(+)-ATPase by immunohistochemistry and western blot (in D. plexippus). Both techniques revealed strong expression of the enzyme in the nervous tissue and indicated weak expression or even absence in other tissues tested. We conclude that instead of target site insensitivity the investigated arctiid species use a different strategy to tolerate cardenolides. Most plausibly, the perineurium surrounding the nervous tissue functions as a barrier which prevents cardenolides from reaching Na(+)/K(+)-ATPase in the ventral nerve cord.