Estimation of the upper limit of the free magnesium concentration measured with Mg-sensitive microelectrodes in ferret ventricular muscle: (1) use of the Nicolsky-Eisenman equation and (2) in calibrating solutions of the appropriate concentration.

Determination of intracellular free magnesium [( Mg]i) with ion-selective Mg microelectrodes filled with the neutral carrier ETH 1117 is critically dependent on the values taken for K and Na in the calibrating solutions, for intracellularly the Mg microelectrodes also respond to these ions. This study was designed to test the influence of K and Na on the measured [Mg]i in ferret ventricular muscle. To estimate the effect of these ions, two methods were used: calculation based on the Nicolsky-Eisenman equation and calibration of the electrodes in solutions containing various concentrations of K and Na. Selectivity coefficients were measured for K and Na by the fixed interference method. While the Nicolsky-Eisenman equation could describe the behavior of an electrode in a given calibrating solution, the effects of changes in either the concentration of K and/or Na could not be predicted by the equation. Comparison between calibrating solutions containing the mean values for K and Na and calibrating solutions containing the lowest measured values for K and Na showed that the mean [Mg]i in ferret ventricle would increase from 0.4 to 0.9 mmol/l. A critical appraisal of the literature values for [Mg]i in muscle tissue suggests that values over 2 mmol/l were either unphysiological or too high for methodological reasons.