UPTAKE AND SEQUESTRATION OF OUABAIN AND OTHER CARDIAC-GLYCOSIDES IN DANAUS-PLEXIPPUS (LEPIDOPTERA, DANAIDAE) - EVIDENCE FOR A CARRIER-MEDIATED PROCESS

Larvae of Danaus plexippus feed almost exclusively on milkweed species of the genus Asclepias, whose characteristic secondary metabolites are cardiac glycosides (CGs). Aposematic last-instar larvae were fed with ouabain and other cardiac glycosides of differing polarities. Time course experiments show that ouabain is sequestered in the integument within 48 hr after feeding, whereas midgut tissue and hemolymph function as transient CG storage compartments. About 63% of ouabain was transferred from larvae to the butterflies, whereas 37% of ouabain was lost with larval and pupal exuviae and with the meconium. The main sites of storage in imagines are wings and integument. If mixtures of CGs are fed to D. plexippus larvae, differential sequestration can be observed: The polar ouabain contributes 58.8% of total CGs, followed by digitoxin (19.6%), oleandrin (10.6%), digoxin (4.9%), digoxigenin (4.6%) and proscillaridin A (1.5%). Thus, uptake and sequestration must be selective processes. Uptake of [H-3]ouabain in vitro by isolated larval midguts was time-, pH-, and temperature-dependent and displayed an activation energy of 49 kJ/mol. Furthermore, the in vitro uptake of ouabain was inhibited (probably competitively) by the structurally similar convalla-toxin. These data provide first evidence that ouabain uptake does not proceed by simple diffusion but with the aid of a carrier mechanism, which would explain the differential cardenolide uptake observed in living larvae.