Extracellular calcium stimulates Na +-dependent putrescine uptake in B16 melanoma cells

Abstract

The regulation of putrescine transport in difluoromethylornithine-treated B16 melanoma cells by extracellular Ca 2+ has been investigated. It was found that physiological concentrations of Ca 2+ were essential for optimum uptake of putrescine and spermidine. Mg 2+, 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 Ca 2+ to 3100 ± 98 pmol/min/mg DNA in the presence of 0.5 mM Ca 2+. There was no change in K m. While Ca 2+ enhanced transport of both putrescine and spermidine it did not affect the uptake of deoxyglucose, thymidine or leucine. Putrescine did not alter Ca 2+ fluxes suggesting that the two cations do not share a common transport system. The effects of Ca 2+ on putrescine uptake appeared to be mediated extracellularly firstly because Ca 2+ did not potentiate putrescine uptake in the presence of A23187 and secondly, because the effects of Ca 2+ were completely inhibited by the lanthanide Tb 3+, which binds to calcium-dependent proteins and does not readily cross biological membranes. Ca 2+ did not affect putrescine transport in the absence of extracellular Na +. Moreover, the rate of putrescine uptake in the absence of Ca 2+ was similar to that in the absence of extracellular Na +. The results from this study indicate that polyamine transport is stimulated by extracellular Ca 2+ and suggest that Ca 2+ is required for activity of the Na +-dependent transporter only. This transporter appears to possess a regulatory binding site for divalent cations.