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Loading rat heart myocytes with Mg 2+ using low‐[Na + ] solutions
Author(s) -
Almulla Hasan A.,
Bush Peter G.,
Steele Michael G.,
Ellis David,
Flatman Peter W.
Publication year - 2006
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2006.109850
Subject(s) - antiporter , extracellular , myocyte , biophysics , chemistry , intracellular , depolarization , ion transporter , membrane potential , membrane , biochemistry , endocrinology , biology
The objective of our study was to investigate how Mg 2+ enters mammalian cardiac cells. During this work, we found evidence for a previously undescribed route for Mg 2+ entry, and now provide a preliminary account of its properties. Changes in Mg 2+ influx into rat ventricular myocytes were deduced from changes in intracellular ionized Mg 2+ concentration ([fMg 2+ ] i ) measured from the fluorescence of mag‐fura‐2 loaded into isolated cells. Superfusion of myocytes at 37°C with Ca 2+ ‐free solutions with both reduced [Na + ] and raised [Mg 2+ ] caused myocytes to load with Mg 2+ . Uptake was seen with solutions containing 5 m m Mg 2+ and 95 m m Na + , and increased linearly with increasing extracellular [Mg 2+ ] or decreasing extracellular [Na + ]. It was very sensitive to temperature ( Q 10 > 9, 25‐‐37°C), was observed even in myocytes with very low Na + contents, and stopped abruptly when external [Na + ] was returned to normal. Uptake was greatly reduced by imipramine or KB‐R7943 if these were added when [fMg 2+ ] i was close to the physiological level, but was unaffected if they were applied when [fMg 2+ ] i was above 2 m m . Uptake was also reduced by depolarizing the membrane potential by increasing extracellular [K + ] or voltage clamp to 0 mV. We suggest that initial Mg 2+ uptake may involve several transporters, including reversed Na + –Mg 2+ antiport and, depending on the exact conditions, reversed Na + –Ca 2+ antiport. The ensuing rise of [fMg 2+ ] i , in conjunction with reduced [Na + ], may then activate a new Mg 2+ transporter that is highly sensitive to temperature, is insensitive to imipramine or KB‐R7943, but is inactivated by depolarization.