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

1. The hormone‐sensitive transport of Mg 2+ into murine S49 lymphoma cells and its relationship to other divalent cation transport systems have been investigated. 2. Mg 2+ influx, measured with 28 Mg 2+ , is saturable with an apparent extracellular ion concentration at half‐maximal influx ( K in ) for Mg 2+ of 330 μM and a maximal influx rate of 360 p‐mole/min.10 7 cells (2·9 n‐mole/min.mg cell protein or a flux rate of about 0·12 p‐mole/sec.cm 2 ). Efflux of Mg 2+ is biphasic with half‐times of 55 and 240 min at 37 °C and is temperature‐sensitive. 3. β‐Adrenergic agonists inhibit influx but not efflux of Mg 2+ in S49 cells. Efflux of Mg 2+ is also unaffected by extracellular [Mg 2+ ] or [Ca 2+ ]. These results imply that the mechanism of the transport system does not involve Mg—Mg exchange. 4. Mn 2+ is a non‐competitive inhibitor of Mg 2+ influx with an inhibition constant, K i , of about 200 μM. The weak inhibition exhibited by Ca 2+ ( K i > 5 mM) is also non‐competitive. La 3+ inhibits Mg 2+ transport half‐maximally at about 100 μM; Ni 2+ , Zn 2+ , Co 2+ and Sc 3+ are all less effective than La 3+ . The Ca 2+ ‐channel blockers cis ‐diltiazem, verapamil, and nifedipine and the monovalent cations Na + and K + also have no effect on Mg 2+ influx. However, increasing the extracellular pH stimulates Mg 2+ influx. 5. Total cellular Mg 2+ is about 85 n‐mole/10 7 cells; however, at apparent isotopic equilibrium with 28 Mg 2+ less than 3% of total cellular Mg 2+ has been exchanged. This indicates that cellular Mg 2+ is highly compartmented and that recently transported Mg 2+ exchanges very slowly with bulk intracellular Mg 2+ . 6. Ca 2+ influx has a K in of 80 μM and is much slower than Mg 2+ influx. V max varied in different experiments from 3 to 15 p‐mole/min.10 7 cells (25‐125 p‐mole/min.mg cell protein). Efflux of Ca 2+ 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 Ca 2+ . Mg 2+ is a competitive inhibitor of Ca 2+ influx ( K i = 3 mM). 7. Two kinetic components of Mn 2+ influx are present with apparent K in s of 4 μM and 100 μM. Maximal influx rates are 5 and 60 p‐mole/min.10 7 cells (40 and 480 p‐mole/min.mg cell protein), respectively. Influx of Mn 2+ is not altered by β‐adrenergic agonist. 8. Uptake of Na + or K + is unaltered by β‐adrenergic stimulation. These data in the S49 lymphoma cell indicate that (a) Mg 2+ is translocated by a transport system independent of those that transport other divalent cations, (b) hormonal inhibition of divalent ion transport is specific for Mg 2+ and (c) cellular Mg 2+ is highly compartmented.