Insects have an astounding ability to cope with environmental challenges. Doug Neufeld and his colleagues have discovered one of the reasons why insects can devour almost anything and emerge unscathed. They reveal that the insect`kidney', the Malpighian tubules, can flush harmful substances like pesticides out of insects' bodies (p. 2227).
Reflecting on the fact that insects happily eat plants that contain toxins or have been sprayed with herbicides and insecticides, Neufeld reasoned that insects must be able to deal with this toxic melange. He suspected that organic anion transporters in insects' Malpighian tubules might play a role in the disposal of environmental toxins. He knew that the fluorescent dye fluorescein is a good tracer for organic anion transport processes, because it is pumped into the Malpighian tubules by these transporters. To show that insects can excrete toxins through organic anion transporters, Neufeld needed to show that a toxin stops the transport of fluorescein into the tubules. If it does, the toxin is either blocking fluorescein transport or is being transported itself in favour of fluorescein, Neufeld explains.
Teaming up with Ross Kauffman and Zachary Kurtz, Neufeld set out to test whether toxins are transported into insects' Malpighian tubules. Examining Malpighian tubules from crickets under a microscope, the team pumped solutions containing fluorescein and each of various test toxins across the tubules. To see if fluorescein was still being transported in the presence of each toxin,they measured how quickly the tubules started glowing, using a photometer to detect how much light was given off. They found that several toxins stopped fluorescein uptake by the tubules, including a plant alkaloid, herbicides and two insecticide metabolites (produced when an insect breaks down an insecticide).
But are these toxins really being transported by the tubules? To find out,Neufeld measured the precise amounts of the two insecticide metabolites in cricket Malpighian tubules, using high performance liquid chromatography to analyse which compounds were present in tubule tissue. Sure enough, both of the insecticide metabolites were present in the tubules. But this didn't convince Neufeld. To be sure that the cells were transporting the metabolites,he killed the tubule cells. He found that only one of the metabolites was still being accumulated in the tubule. `This metabolite probably just sticks to the tubule, but is not actually transported across the tubule cell,'Neufeld concludes. But the other metabolite was only accumulated when the tubules were alive, so crickets' tubule cells do excrete this metabolite.
Neufeld reveals an intriguing application of these findings. If insects can purge insecticides, they can evade eradication. But if we can entice insects to gobble up an inhibitor that blocks insecticide metabolite transport so the insects can't expel the toxins anymore, `we could make insecticides much more effective,' Neufeld speculates.