| Abstrakt: |
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 muM 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 degrees C and is temperature-sensitive.3. beta-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 muM. 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 muM; 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 muM 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 muM and 100 muM. 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 beta-adrenergic agonist.8. Uptake of Na(+) or K(+) is unaltered by beta-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. |
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