Uptake and sequestration of ouabain and other cardiac glycosides inDanaus plexippus (Lepidoptera: Danaidae): Evidence for a carrier-mediated process

Abstract

Larvae ofDanaus plexippus feed almost exclusively on milkweed species of the genusAsclepias, 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 toD. 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 [(3)H]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 convallatoxin. 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.

Selective transport systems mediate sequestration of plant glucosides in leaf beetles: A molecular basis for adaptation and evolution

“…To account for the high selectivity of the transport processes, we postulate a network of hydrogen bonds between the transport protein and the hydroxyl groups of the glucose and the aglycon, which together seem to account for selection and transportation of 10. It is unknown whether the two complex transport systems in the gut and the glandular system are identical (30). Initial results with gut tissue from iridoid-producing insects (P. cochleariae and G. viridula) demonstrate a comparative selectivity for the uptake of 1 into the epithelial cells to that observed for the living insect (S. Discher and W.B., unpublished data).…”

Plant chemistry and insect sequestration

“…D. plexippus readily sequesters more polar CGs such as ouabain, whereas the more lipophilic CGs and aglycones (e.g. uzarigenin) are found in larvae only to a minor extent (Seiber et al 1980;Frick and Wink 1995). The latter probably simply diffuse across membranes and are then metabolised to facilitate storage.…”

Sequestration of phorbolesters by the aposematically coloured bug Pachycoris klugii (Heteroptera: Scutelleridae) feeding on Jatropha curcas (Euphorbiaceae)

“…Examples for a storage in insect integuments include pyrrolizidine alkaloids in Creatonotos and other arctiids (Egelhaaf et al 1990;von Nickisch-Rosenegk et al 1990;Wink et al 1990), quinolizidine alkaloids in Uresiphita re6ersalis (Fam. Pyralidae) or cardiac glycosides in Danaus plexippus (Frick & Wink 1995) or the bug Oncopeltus fasciatus (Detzel & Wink 1991). Because the phorbol esters of J. curcas show a wide and acute toxicity against vertebrates and insects (see above), we suggest that P. klugii utilises the sequestered phorbol esters as chemical defence compounds against predators.…”