Intracellular free Mg²⁺ concentration in skeletal muscle fibres of frog and crayfish

Abstract

The free intracellular Mg²⁺ concentration ([Mg²⁺]_i) was investigated in frog sartorius and crayfish phasic and tonic skeletal muscle fibres, using a new Mg²⁺-sensitive microelectrode based on the ionophore ETH 5214 [Hu et al. (1989) Anal Chem 61:574–576]. In Ringer solution containing 0.5 mmol/l MgCl₂, the mean [Mg²⁺]_i of the frog muscle fibres was 1.3 mmol/l. In phasic crayfish muscle fibres, [Mg²⁺]_i was about twice as high (mean 3.5 mmol/l) as in tonic fibres (mean 1.5 mmol/l), measured in van Harreveld solution containing 1.2 mmol/l MgCl₂. Long-lasting (3–12 h) incubation of frog skeletal muscle fibres in Na⁺-free solution produced a reversible increase of [Mg²⁺]_i by a factor of about 1.7. A tenfold rise of extracellular Mg²⁺ led to an increase in [Mg²⁺]_i in the presence as well as in the absence of Na⁺. In these experiments, mean [Mg²⁺]_i values of 3.2 mmol/l were never exceeded. Thus, [Mg²⁺]_i remained at least 60 times lower than predicted from a passive distribution across the cell membrane. The results suggest the existence of a Na⁺-dependent and a Na⁺-independent Mg²⁺ extrusion mechanism, which is regulated by actual Mg²⁺ concentrations.