Paraquat uptake in the cultured gastrointestinal epithelial cell line, IEC-6

Although the major target organ of paraquat toxicity is the lung, the primary route of absorption of the herbicide is across the gastrointestinal epithelium. Thus, uptake of paraquat was investigated in the rat duodenal crypt cell line, IEC-6. The herbicide was toxic; as measured by cell number, the LD50 was 75 microM. Using 2 microM [14C]paraquat as tracer, uptake of the herbicide was found to be slow but linear over 24 hr at 37 degrees C. No accumulation was observed at 4 degrees C. Inhibition studies showed paraquat inhibited both putrescine and spermidine uptake after only a 15-min incubation. The Km for putrescine was increased when incubated in the presence of paraquat whereas Vmax was not changed, suggesting a competitive mode of inhibition. In the reverse situation, putrescine also inhibited paraquat uptake (IC50 10 microM). Paraquat uptake was reduced in a dose-dependent manner by the putative calmodulin antagonist W-7, but was not affected by KN-62, a Ca2+/calmodulin kinase II specific inhibitor. These results provide evidence that paraquat uptake occurs through the polyamine transport system in IEC-6 cells. Furthermore, this process is modulated by intracellular events involving a major signaling protein, Ca2+/calmodulin.