Intracellular free magnesium in frog skeletal muscle fibres measured with ion‐selective micro‐electrodes.

Abstract

1. Intracellular free Mg2+ concentration ([Mg2+]i) was measured in frog skeletal muscle fibres, using Mg2+-selective micro-electrodes based on the neutral ligand ETH-1117. 2. In calibration solutions the electrodes showed significant interference from K+, and to a lesser extent from Na+, at concentrations found intracellularly. Therefore, in order to calibrate the electrodes properly, it was necessary first to obtain an accurate value for intracellular free Na+ and K+ concentrations ([Na+]i and [K+]i), using the appropriate liquid ion exchanger micro-electrodes. In fibres from muscles maintained in Ringer solution, the mean value for [Na+]i was 6.2 .+-. 0.4 mM (S.E. of mean; n = 20 fibres in five muscles), while [K+]i was 104 .+-. 1.7 mM (range 83-122 mM; n = 25 fibres in eight muscles). 3. Due to the substantial variability found for [K+]i, not only between fibres from different muscles, but also between fibres belonging to the same muscle, it was necessary to measure [Mg2+]i and [K+]i simultaneously in the same fibre to determine as accurately as possible the degree of K+ interference on Mg2+-selective microelectrode response. In nineteen fibres from six muscles maintained in Ringer solution, the mean [K+]i was 91.7 .+-. 2.7 mM (range 71-110 mM), while the mean [Mg2+]i was 0.80 .+-. 0.07 mM (range 0.2-1.2 mM). The mean resting potential was -79.3 .+-. 0.4 mV (S.E. of mean). 4. In fifteen fibres from four muscles equilibrated in Ringer solution containing 0.5 mM-Mg2+, the mean [K+]i was 115.5 .+-. 0.1 mM (range 97-129 mM) and the mean [Mg2+]i measured simultaneously in the same fibres was 1.69 .+-. 0.21 mM (range 0.2-2.7 mM). The mean resting potential was -83 .+-. 0.cntdot.7 mV. The mean [K+]i and [Mg2+]i found in these fibres was significantly higher (P < 0.001) than those measured in fibres from muscles maintained in standard Ringer solution (i.e. without external Mg2+). Possible explanations for this finding are discussed. 5. Whether in the presence (0.5 mM) or in the absence of external Mg2+, our values for [Mg2+]i are distinctly lower than those previously reported by others, using the same type of Mg2+-selective micro-electrodes but calibrated simply from assumptions about the actual level of K+ and Na+ interference on Mg2+-selective micro-electrode response. 6. If Mg2+ was in electrochemical equilibrium across membrane, the mean [Mg2+]i in the presence of a physiological concentration of external Mg2+ (0.5 mM) would be more than 100 times the actual value. Therefore our results confirm that Mg2+ is not passively distributed across the muscle membrane and an outwardly directed extrusion mechanism must exist to keep [Mg2+]i low.