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 (pMg_i=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.