Hormone‐sensitive magnesium transport in murine S49 lymphoma cells: characterization and specificity for magnesium

Abstract

The hormone-sensitive transport of Mg2+ into murine S49 lymphoma cells and its relationship to other divalent cation transport systems were investigated. Mg2+ influx, measured with 28Mg2+, is saturable with an apparent extracellular ion concentration at half-maximal influx (Kin) for Mg2+ of 330 .mu.M and a maximal influx rate of 360 pmol/min .cntdot. 107 cells (2.9 nmol/min .cntdot. mg cell protein or a flux rate of about 0.12 pmol/s .cntdot. cm2). Efflux of Mg2+ is biphasic with half-times of 55 and 240 min at 37.degree. C and is temperature-sensitive. .beta.-Adrenergic agonists inhibit influx but not efflux of Mg2+ in S49 cells. Efflux of Mg2+ is also unaffected by extracellular [Mg2+] or [Ca2+]. The mechanism of the transport system apparently does not involve Mg-Mg exchange. Mn2+ is a non-competitive inhibitor of Mg2+ influx with an inhibition constant, Ki, of about 200 .mu.M. The weak inhibition exhibited by Ca2+ (Ki > 5 mM) is also non-competitive. La3+ inhibits Mg2+ transport half-maximally at about 100 .mu.M; Ni2+, Zn2+, Co2+ and Sc3+ are all less effective than La3+. The Ca2+-channel blockers cis-diltiazem, verapamil and nifedipine and the monovalent cations Na+ and K+ also have no effect on Mg2+ influx. Increasing the extracellular pH stimulates Mg2+ influx. Total cellular Mg2+ is about 85 nmol/107 cells; however, at apparent isotopic equilibrium with 28Mg2+ less than 3% of total cellular Mg2+ was exchanged. This indicates that cellular Mg2+ is highly compartmented and that recently transported Mg2+ exchanges very slowly with bulk intracellular Mg2+. Ca2+ influx has a Kin of 80 .mu.M and is much slower than Mg2+ influx. Vmax varied in different experiments from 3-15 pmol/min .cntdot. 107 cells (25-125 pmol/min .cntdot. mg cell protein). Efflux of Ca2+ is biphasic with half-times of 22 and 200 min and is temperature-sensitive. Hormonal stimulation has no effect on either influx or efflux of Ca2+. Mg2+ is a competitive inhibitor of Ca2+ influx (Ki = 3 mM). Two kinetic components of Mn2+ influx are present with apparent Kin of 4 .mu.M and 100 .mu.M. Maximal influx rates are 5 and 60 pmol/min .cntdot. 107 cells (40 and 480 pmol/min .cntdot. mg cell protein), respectively. Influx of Mn2+ is not altered by .beta.-adrenergic agonist. Uptake of Na+ or K+ is unaltered by .beta.-adrenergic stimulation. Mg2+ is apparently translocated by a transport system independent of those that transport other divalent cations; hormonal inhibition of divalent ion transport is specific for Mg2+ and cellular Mg2+ is highly compartmented.