![]() ![]() For insects, one has been characterized as a multi-alkaloid transporter on the basis of mutually competitive transport of nicotine, atropine and morphine in the Malpighian tubules of Rhodnius prolixus ( Maddrell and Gardiner, 1976), a blood-sucking bug (Hemiptera) unlikely to encounter any of these compounds in nature. The transporters responsible appear to be few in number but broad in specificity. In both systems, non-specific filtration is supplemented by active transport mechanisms. The insect Malpighian tubule is often compared with the vertebrate kidney, since both perform the role of eliminating wastes from the body fluid. ![]() Taken together, these results are consistent with the idea that the active excretion of nicotine and other alkaloids by the tobacco hornworm is mediated by a P-glycoprotein-like mechanism. Verapamil was effective at reducing vinblastine transport when applied to the basal side alone, but not when applied to the apical side alone. Nicotine transport was inhibited by atropine (3 mmol l −1), while nicotine (⩾50 μmol l −1) significantly reduced vinblastine transport. Vectorial transport of vinblastine and nicotine was eliminated by 25 μmol l −1 verapamil (a P-glycoprotein inhibitor) and was not dependent on the presence of a transepithelial electrical potential. ![]() Although these ratios provide conservative estimates of the physiological ability of Malpighian tubules to move compounds from blood to lumen, tubules concentrated nicotine 10-fold from an initial bath concentration of 0.5 mmol l −1 and vinblastine threefold (from an initial concentration of 1 μmol l −1). The luminal (apical) contents were then flushed, and lumen-to-bath ratios were measured. Isolated tubules were cannulated and exposed to radiolabelled forms of either nicotine (5 min exposure) or the P-glycoprotein substrate vinblastine (60 min exposure) in the bathing (basal surface) fluid. ![]() We have examined the accumulative transport properties of the Malpighian (excretory) tubules of the tobacco hornworm Manduca sexta to test the hypothesis that a P-glycoprotein-like multidrug transporter is active and is responsible for the excretion of dietary nicotine in this tissue. ![]()
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