Putrescine uptake and release by colon cancer cells

We have investigated the uptake and release of [3H]putrescine by a human colon adenocarcinoma cell line (LoVo) maintained on filter inserts. This culture system permits the cells to develop morphological polarity and provides separate access to the basolateral and apical surfaces of the cells. [3H]putrescine was taken up more readily by the basolateral than by the apical side of the cells. [3H]putrescine uptake could be stimulated greater than 300 times by either 10 mM asparagine or 10% fetal bovine serum. [3H]putrescine was accumulated to a concentration gradient of approximately 300-fold; uptake could be inhibited 50% by 7.5 microM unlabeled putrescine and was not dependent on Na+. The release of [3H]putrescine into the apical medium was inhibited by asparagine or fetal bovine serum. Usually, less than one-thousandth of the [3H]putrescine taken up into the cells was released into the apical medium. Release of [3H]putrescine did not correspond to the accumulation of [14C]-inulin in the apical medium. For these reasons we concluded that putrescine release was not simply passive leakage but was responsive to intracellular demand. The [3H]putrescine taken up by the cells as well as that released into the apical medium was greater than 90% unmetabolized at 4h.