Modulation of melphalan uptake in murine L5178Y lymphoblasts in vitro by changes in ionic environment

The alkylating agent melphalan is actively transported in mammalian cells by two amino acid transport carriers: the sodium-dependent carrier with substrate preference for alanine-serine-cysteine (system ASC), and a sodium-independent carrier with preference for leucine (system L). The effect of altering the ionic environment of murine L5178Y lymphoblasts was investigated in order to determine not only the direct effects of hydrogen and calcium ions on these transport systems, but also the indirect effects of agents or modulators known to alter intracellular calcium. Melphalan transport followed a bell-shaped distribution curve over a pH range from 3 to 9 with a pH optimum of 4.3 and 4.6 for transport by systems ASC and L, respectively. Those agents that could cause a decrease in cytosolic calcium such as the calcium channel blockers verapamil, diltiazem and nitrendipine, the calcium chelator (ethyleneglycol-bis-(beta-aminoethylether) N,N,N',N'-tetraacetic acid (EGTA) and reduction of pH were found to augment melphalan uptake, whereas conditions that would elevate intracellular calcium such as the calcium ionophore A23187, the calcium channel agonist (-) Bay K 8644, elevation of extracellular calcium and the calcium pump inhibitor trifluoperazine were all found to decrease melphalan uptake. These findings suggest that modification of ionic environment directly or indirectly by agents known to alter intracellular calcium can modulate melphalan uptake.