Spermidine uptake by type II pneumocytes: Interactions of amine uptake pathways

Uptake of exogenous spermidine by type II pulmonary epithelial cells in primary culture is inhibited by several amines. The present studies further detail the interaction of those alternative substrates with the pathway for spermidine (SPD) transport. Transport activity was measured on the first day of primary culture in type II cells isolated by elastase digestion, followed by density gradient centrifugation and differential adherence in vitro. Spermidine uptake was inhibited in a concentration-dependent manner by the polyamine putrescine (PUTR) and spermine (SPM), by the drug methylglyoxal bis-(guanylhydrazone) (MGBG), and by the herbicide paraquat (PQ). The order of effectiveness of these competitors was SPM > PUTR ≃ MGBG >> PQ. The kinetics of inhibition by SPM were mixed, with an increase in the apparent Michaelis constant and a reduction in the maximal velocity of the SPD uptake pathway. Cellular uptake of SPD and PUTR was inhibited by replacement of extracellular sodium with choline or lithium (PUTR > SPD), but SPM uptake was unaffected. PQ appeared to interact with the polyamine transport pathway with low affinity. Compared with SPD and SPM, PQ was most effective as an inhibitor of PUTR uptake, and like PUTR, uptake of the herbicide was sharply inhibited as extracellular sodium was reduced. These observations suggest the presence of diverse pathways for uptake of exogenous polyamines by type II pulmonary epithelial cells and indicate that PQ probably enters the cells by the sodium-dependent transport system favored by PUTR.