Evidence for a Cardenolide Carrier in Oncopeltus fasciatus (Dallas) (Insecta: Hemiptera)

Oncopeltus fasciatus, Chemical Defence, Cardenolide Sequestration, Carrier-Mediated Uptake, Cardenolide Carrier Oncopeltus fasciatus (Hemiptera) is morphologically and physiologically adapted to seques­ ter cardiac glycosides for its chemical defence against predators. As a prerequisite for carde­ nolide utilization this bug has to take up and store the dietary cardenolides. Using isolated midguts it could be shown experimentally that the resorption of [3H]digoxin is time-, and temperaturedependent. The hydrogen ion concentration for uptake is optimal between pH 5.5 and 7 and an activation energy of 45 to 49 kJ/mol can be derived from an Arrhenius plot. Uptake can be inhibited competitively by the polar cardiac glycoside convallatoxin. Sulfhydryl group reagents (N-ethylmaleimide), membrane destabilizing compounds and respira­ tory chain inhibitors reduce digoxin resorption, as do inhibitors of sugar transport (e.g., phloridzin). The experimental data thus provide evidence for a cardenolide carrier in midgut cells of O. fasciatus.