Spermidine uptake by type II pulmonary epithelial cells in primary culture

The transport pathway for the polyamine spermidine (SPD) was characterized in primary isolates of type II pulmonary epithelial cells from rat lungs. [¹⁴C]spermidine was accumulated by type II cells via a temperature-, sodium-, and concentration-dependent saturable pathway, with an apparent K(m) of 0.48 μM and a maximum velocity (V(max)) of 0.32 pmol·μg DNA^-1·min^-1. SPD uptake was inhibited by gramicidin and by reduced extracellular sodium but was unaffected by α-aminoisobutyric acid (AIB), which entered the cells by a similar saturable pathway. Uptake of SPD also was inhibited by the exogenous polyamines putrescine (PUTR) and spermine (SPM), as well as by the methylglyoxal bis(guanylhydrazone) (MGBG) and by paraquat (PQ). The order of potency of these inhibitors was SPM > PUTR = MGBG >> PQ. The absence of serum reduced the V(max) of the system slightly but had no effect on the apparent K(m). In contrast, after 3 days in primary cell culture, the kinetics of SPD transport were altered by decreases in both the K(m) and V(max) of the uptake process. These observations indicate that type II pulmonary epithelial cells exhibit a pathway of polyamine uptake with general characteristics similar to those observed previously in intact lung tissue and other cell types.