Uptake,efflux and metabolism of the polyamine putrescine in rabbit lung slices

The herbicide paraquat is a selective pulmonary toxin in many mammals, including man, and its pulmonary toxicity has been attributed to selective uptake by a polyamine transport system in lung. In the present study, we investigated the characteristics of this transport process in rabbit lung slices. [¹⁴C]Putrescine was accumulated by both saturable and non-saturable processes and the accumulated putrescine was non-effluxable over 60 min. The saturable component was inhibited by spermine and paraquat. Moreover, uptake studies in Na⁺-deficient medium indicated that the lack of Na⁺ may selectively enhance uptake via the non-saturable process. The two components also differed in the metabolic fate of accumulated substrate. At 0.6 μM putrescine, where the saturable process predominated, 98% of the ¹⁴C in the perchloric acid-soluble fraction of tissue homogenates was present as putrescine, whilst 3% of the accumulated substrate was found in the acid-insoluble fraction. With 500 μM putrescine, where the non-saturable process predominated, 82% of the ¹⁴C in the acid-soluble fraction was present as putrescine and 15% of accumulated putrescine was found in the acid-insoluble fraction. The acid-insoluble ¹⁴C was localised mainly in the 700 g and 4500 g pellets obtained after homogenising the tissue. We conclude that there are two components to putrescine uptake in rabbit lung slices, both of an apparently irreversible nature. We suggest that the components represent compartmentalisation of putrescine in selective pulmonary cell-types or separate subcellular organelles. The observed metabolism and covalent binding of putrescine appeared to be associated with the non-saturable component only.