Extracellular calcium stimulates Na(+)-dependent putrescine uptake in B16 melanoma cells.
The regulation of putrescine transport in difluoromethylornithine-treated B16 melanoma cells by extracellular Ca2+ has been investigated. It was found that physiological concentrations of Ca2+ were essential for 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/min/mg DNA in the absence of Ca2+ to 3100 +/0 98 pmol/min/mg DNA in the presence of 0.5 mM Ca2+. There was no change in Km. While Ca2+ enhanced transport of both putrescine and spermidine it did not affect the uptake of deoxyglucose, thymidine or leucine. Putrescine did not alter Ca2+ fluxes suggesting that the two cations do not share a common transport system. The effects of Ca2+ on putrescine uptake appeared to be mediated extracellularly firstly because Ca2+ did not potentiate putrescine uptake in the presence of A23187 and secondly, because the effects of Ca2+ were completely inhibited by the lanthanide Tb3+, which binds to calcium-dependent proteins and does not readily cross biological membranes, Ca2+ did not affect putrescine transport in the absence of extracellular Na+. Moreover, the rate of putrescine uptake in the absence of Ca2+ was similar to that in the absence of extracellular Na+. The results 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 possess a regulatory binding site for divalent cations.
Minchin
R F
RF
Department of Pharmacology, University of Western Australia, Nedlands, Australia.
Martin
R L
RL
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism / Q000493:pharmacokinetics / Q000494:pharmacology*; D002352:Carrier Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D008545:Melanoma / Q000188:drug therapy / Q000378:metabolism* / Q000473:pathology; D051379:Mice; D011073:Polyamines / Q000378:metabolism; D011700:Putrescine / Q000493:pharmacokinetics*; D012964:Sodium / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D002352:Carrier Proteins; D011073:Polyamines; D012964:Sodium; D002118:Calcium; D011700:Putrescine