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 ar e cardiac glycosides (CGs). Aposematic last-instar larvae were fed wit h ouabain and other cardiac glycosides of differing polarities. Time c ourse experiments show that ouabain is sequestered in the integument w ithin 48 hr after feeding, whereas midgut tissue and hemolymph functio n as transient CG storage compartments. About 63% of ouabain was trans ferred from larvae to the butterflies, whereas 37% of ouabain was lost with larval and pupal exuviae and with the meconium. The main sites o f storage in imagines are wings and integument. If mixtures of CGs are fed to D. plexippus larvae, differential sequestration can be observe d: The polar ouabain contributes 58.8% of total CGs, followed by digit oxin (19.6%), oleandrin (10.6%), digoxin (4.9%), digoxigenin (4.6%) an d proscillaridin A (1.5%). Thus, uptake and sequestration must be sele ctive processes. Uptake of [H-3]ouabain in vitro by isolated larval mi dguts was time-, pH-, and temperature-dependent and displayed an activ ation energy of 49 kJ/mol. Furthermore, the in vitro uptake of ouabain was inhibited (probably competitively) by the structurally similar co nvalla-toxin. These data provide first evidence that ouabain uptake do es not proceed by simple diffusion but with the aid of a carrier mecha nism, which would explain the differential cardenolide uptake observed in living larvae.