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Intracellular free magnesium and its regulation, studied in isolated ferret ventricular muscle with ion‐selective microelectrodes

Intracellular free magnesium ([Mg2+]i) was measured in isolated ferret papillary muscles using ion‐selective microelectrodes filled with the new magnesium sensor ETH 5214. This new sensor, unlike its predecessor ETH 1117, does not react to marked changes in K+, Na+ or pH. Reducing Ca2+ from 20 microM to around 10 nM also did not affect the response so these electrodes are ideally suited to study intracellular Mg2+ and its regulation. The mean value for the [Mg2+]i from thirty‐two experiments (forty‐two impalements) was 0.85 mM, confirming previous estimates from this laboratory. Intracellular Mg2+ is not passively distributed and the possibility that Mg2+ is transported out of the cell by a Na(+)‐Mg2+ exchanger was investigated. An increase in [Mg2+]o caused an increase in [Mg2+]i, as did stepwise reduction in the [Na+]o. However, this increase in [Mg2+]i on Na+ reduction also occurred in Mg2(+)‐free solution suggesting that the increase in [Mg2+]i was due to the increase in intracellular Ca2+ on Na+ reduction. Moreover, increasing [Na+]i by strophanthidin did not change the [Mg2+]i and on increasing [Mg2+]o there was no reduction in the [Na+]i. Blocking ATP production lead to small increases in the [Mg2+]i. These results are not consistent with a Na(+)‐Mg2+ exchanger as being the main outward transport mechanism for Mg2+ in this tissue.