Measurement and control of intracellular magnesium ion concentration in guinea pig and ferret ventricular myocardium.

Intracellular magnesium and sodium ionic concentrations have been measured in mammalian ventricular myocardium using ion-selective micro-electrodes. The values for [Na+]i and [Mg2+]i are 9.5 +/- 0.3 and 2.4 +/- 0.2 mM, respectively. Increasing extracellular [Mg] or decreasing extracellular [Na] both caused a rise of [Mg2+]i and a fall of [Na+]i. Superfusion with ouabain caused a small rise of [Mg2+]i as well as a rise of [Na+]i. Reduction of extracellular [Mg] or cell depolarisation had no effect on [Mg2+]i. A possible hypothesis to explain these results is proposed, namely that the transmembrane Na+ and Mg2+ gradients are coupled such that an alteration of one gradient will affect the other. However, the relative stability of [Mg2+]i suggests that it is also affected by other factors, such as intracellular buffering of the ion.