Magnesium homeostasis in cardiac cells.
Several aspects of Mg2+ homeostasis were investigated in cultured chicken heart cells using the fluorescent Mg2+ indicator, FURAPTRA. The concentration of cytosolic Mg2+ ([Mg2+]i) is 0.48 +/- 0.03 mM (n = 31). To test whether a putative Na/Mg exchange mechanism controls [Mg2+]i below electrochemical equilibrium, we manipulated the Na+ gradient and assessed the effects on [Mg2+]i. When extracellular Na+ was removed, [Mg2+]i increased; this increase was not altered in Mg-free solutions, but was attenuated in Ca-free solutions. A similar increase in [Mg2+]i, which was dependent upon extracellular Ca2+, was observed when intracellular Na+ was raised by inhibiting the Na/K pump with ouabain. These results do not provide evidence for Na/Mg exchange in heart cells, but they suggest that Ca2+ can modulate [Mg2+]i. In addition, removing extracellular Na+ caused a decrease in intracellular pH (pHi), as measured by pH-sensitive microelectrodes, and this acidification was attenuated when Ca2+ was also removed from the solution. These results suggest that Ca2+ and H+ interact intracellularly. Since changes in the Na+ gradient can also alter pHi, we questioned whether pH can modulate [Mg2+]i. pHi was manipulated by the NH4Cl prepulse method. NH4(+)-evoked changes in pHi, as measured by the fluorescent indicator BCECF, were accompanied by opposite changes in [Mg2+]i; [Mg2+]i changed by -0.16 mM/unit pH. These NH4(+)-evoked changes in [Mg2+]i were not caused by movements of Mg2+ or Ca2+ across the sarcolemma or by changes in cytosolic Ca2+. Additionally, pHi was manipulated by changing extracellular pH (pHo).(ABSTRACT TRUNCATED AT 250 WORDS)
Freudenrich
C C
CC
Department of Cell Biology, Duke University Medical Center, Durham, NC 27710.
Murphy
E
E
Liu
S
S
Lieberman
M
M
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D002645:Chickens; D003593:Cytoplasm / Q000378:metabolism; D006706:Homeostasis; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism*; D008564:Membrane Potentials; D009206:Myocardium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism
D012964:Sodium; D008274:Magnesium; D002118:Calcium