Free Intracellular Magnesium Concentration in Ferret Ventricular Muscle Measured With Ion Selective Micro‐electrodes

The free Mg concentration ([Mg]i) was measured in ferret ventricular muscle using single-barrelled Mg-sensitive micro-electrodes. The electrodes were calibrated in solutions containing the [K]i and [Na]i measured in this tissue. In thirteen measurements from seven experiments conforming to strict criteria for calibration and penetration the estimated mean [Mg]i was 0.4 mmol/l. This is to be regarded as an upper limit since it was estimated by linear extrapolation of the calibration curve.