Gaertner L S, Murray C L, Morris C E
University of Ottawa, Department of Biology and Neurosciences, Loeb Research Institute, Ottawa Hospital, Ottawa, Ontario, Canada K1Y 4E9.
J Exp Biol. 1998 Sep;201(Pt 18):2637-45. doi: 10.1242/jeb.201.18.2637.
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. 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. The luminal (apical) contents were then flushed, and lumen-to-bath ratios were measured. 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 micromol l-1). Vectorial transport of vinblastine and nicotine was eliminated by 25 micromol l-1 verapamil (a P-glycoprotein inhibitor) and was not dependent on the presence of a transepithelial electrical potential. Nicotine transport was inhibited by atropine (3 mmol l-1), while nicotine (> or = 50 micromol l-1) significantly reduced vinblastine transport. Verapamil was effective at reducing vinblastine transport when applied to the basal side alone, but not when applied to the apical side alone. 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.
我们研究了烟草天蛾马氏（排泄）管的累积转运特性，以检验一种类似P - 糖蛋白的多药转运体在该组织中具有活性并负责排泄膳食尼古丁这一假说。将分离出的小管插管，并使其暴露于灌注（基底表面）液中放射性标记形式的尼古丁（暴露5分钟）或P - 糖蛋白底物长春新碱（暴露60分钟）。然后冲洗管腔（顶端）内容物，并测定管腔与灌注液的比率。尽管这些比率提供了对马氏管将化合物从血液转运至管腔的生理能力的保守估计，但小管能将尼古丁从初始灌注浓度0.5 mmol·l⁻¹浓缩10倍，将长春新碱浓缩3倍（从初始浓度1 μmol·l⁻¹）。25 μmol·l⁻¹维拉帕米（一种P - 糖蛋白抑制剂）消除了长春新碱和尼古丁的向量转运，且其不依赖于跨上皮电势的存在。阿托品（3 mmol·l⁻¹）抑制尼古丁转运，而尼古丁（≥50 μmol·l⁻¹）显著降低长春新碱转运。单独应用于基底侧时，维拉帕米能有效降低长春新碱转运，但单独应用于顶端侧时则无效。综上所述，这些结果与烟草天蛾对尼古丁和其他生物碱的主动排泄是由一种类似P - 糖蛋白的机制介导的观点一致。
