Skeletal Muscle Energetics and Mechanics: The Effects of Magnesium on Resting and Contracting Muscle as Studied by Phosphorus Magnetic Resonance Spectroscopy

Abstract

Although much is known about the role of calcium in skeletal muscle mechanics and energetics, little is known about the role of magnesium, especially in intact muscle. This work studies skeletal muscle energetics and mechanics using MR spectroscopy (MRS) with particular reference to the role of magnesium. The accuracy and precision of $\sp{31}$P MRS as a technique for this study was evaluated. A servocontrol system was implemented such that isolated frog skeletal muscles could be made to undergo various isometric or isotonic contractions during acquisition of $\sp{31}$P MRS data. Frog muscles were studied in various temperature and bathing solution environments, and the spectroscopic changes were related to ion flux, metabolic inhibition, and the contractile properties of the muscle.

As experimental conditions were changed, the areas and positions of peaks in the $\sp{31}$P MR spectra were affected. Temperature changes and metabolic inhibition affected the T$\sb1$ (longitudinal relaxation) time constant of the phosphorus compounds and therefore the peak areas. When there was a change in the magnesium levels in the bathing solution for an extended period of time, the $\beta$-ATP peak position changed indicating a change in the free intracellular magnesium level.

Sartorii were exposed to 1 mM Mg-Ringer's while a paired set of muscles were exposed to a Mg-free solution. Muscles exposed to the Mg-free solution initially produced higher isometric force, but also consumed phosphocreatine (PCr) faster and fatigued more quickly than the control muscles. The economy or efficiency of contraction was not affected.