Abstract
Insect Malpighian tubules actively transport a variety of xenobiotics, and it has been proposed that P-glycoprotein (P-gp), or the multidrug transporter, is involved. To test this idea, we observed the interaction of known P-gp substrates with isolated, living Malpighian tubules from tobacco hornworm (Manduca sexta) larvae. Specifically, the fluorescent drugs daunomycin, rhodamine 123, acridine orange and Hoechst 33342 were applied to the basal side of tubules (proximal portion) in a well of fluid on a coverslip; the subsequent distribution of the drugs was monitored by laser scanning confocal microscopy. Contrary to expectation, none of the drugs appeared in the lumen even after 1-2 h of incubation, although the cells of the tubule were intensely stained within 1 min. For daunomycin, neither verapamil, a P-gp inhibitor, nor nicotine, an alkaloid which appears to be transported by a P-gp-like mechanism in this species, had any effect on the pattern of staining. In sharp contrast to the fast and intense staining of Malpighian tubules, portions of muscle, nerve cord and body fat showed only light staining with daunomycin, and only after prolonged periods. The results suggest that for some drugs, Malpighian tubules act as xenobiotic scavengers, and that this property is unrelated to P-gp-mediated transport.