EXTRACELLULAR CALCIUM STIMULATES NA-DEPENDENT PUTRESCINE UPTAKE IN B16 MELANOMA-CELLS()

The regulation of putrescine transport in difluoromethylornithine-trea ted B16 melanoma cells by extracellular Ca2+ has been investigated. It was found that physiological concentrations of Ca2+ were essential fo r optimum uptake of putrescine and spermidine. Mg2+, albeit at higher concentrations, also could potentiate polyamine transport. The maximum rate of putrescine uptake increased from 1698 +/-: 67 pmol/min/mg DNA in the absence of Ca2+ to 3100 +/- 98 pmol/min/mg DNA in the presence of 0.5 mM Ca2+. There was no change in K-m. While Ca2+ enhanced trans port of both putrescine and spermidine it did not affect the uptake of deoxyglucose, thymidine or leucine. Putrescine did not alter Ca2+ flu xes suggesting that the two cations do not share a common transport sy stem. The effects of Ca2+ on putrescine uptake appeared to be mediated extracellularly firstly because Ca2+ did not potentiate putrescine up take in the presence of A23187 and secondly, because the effects of Ca 2+ were completely inhibited by the lanthanide Tb3+, which binds to ca lcium-dependent proteins and does not readily cross biological membran es. Ca2+ did not affect putrescine transport in the absence of extrace llular Na+. Moreover, the rate of putrescine uptake in the absence of Ca2+ was similar to that in the absence of extracellular Na+. The resu lts from this study indicate that polyamine transport is stimulated by extracellular Ca2+ and suggest that Ca2+ is required for activity of the Na+-dependent transporter only. This transporter appears to posses s a regulatory binding site for divalent cations. (C) 1997 Elsevier Sc ience Ltd.