INTRACELLULAR FREE MAGNESIUM IN FROG SKELETAL-MUSCLE STUDIED WITH A NEW TYPE OF MAGNESIUM-SELECTIVE MICROELECTRODE - INTERACTIONS BETWEEN MAGNESIUM AND SODIUM IN THE REGULATION OF [MG]I

The application of a new type of intracellular magnesium-selective microelectrode based on the neutral carrier ETH5214 to measure intracellular free magnesium ([Mg]i) in frog skeletal muscle fibers is reported. At room temperature (18-20° C) the average values for [Mg]i was 0.93 mmol/l (pMgi=3.03±0.42, SD; n = 38 experiments). The regulation of [Mg]i was studied by measuring [Mg]i and [Na]i with ion-selective microelectrodes during alterations of the membrane potential and the transmembrane sodium and magnesium gradients. Depolarization by increasing external [K] from 2.5 mmol/l to 12.5 mmol/l did not significantly influence [Mg]i. Increasing extracellular [Mg] from 1 mmol/l to 10 and 20 mmol/l caused a concentration-dependent rise in [Mg]i and a decrease in [Na]i, whereas removal of external magnesium did not affect [Mg]i. Removal of external [Na] caused an increase in [Mg]i and a decrease of [Na]i. The results show that [Mg]i in frog skeletal muscle is not in thermodynamic equilibrium and suggest that a Na/Mg exchange mechanism may be involved in maintaining low levels of [Mg]i. © 1990 Springer-Verlag.