Intracellular Mg 2+ diffusion within isolated rat skeletal muscle fibers

Abstract

Intracellular free magnesium concentration ([Mg 2+] i) was measured in enzymatically isolated rat skeletal muscle fibers using the fluorescent dye mag-indo-1. The change in [Mg 2+] i produced by a local intracellular microinjection of magnesium pidolate (magnesium pyrrolidone carboxylate) was measured at a given distance from the injection site. In one series of experiments this protocol was tested on isolated fibers that were completely embedded into silicone grease: under these conditions, the injection produced an increase in [Mg 2+] i that reached a steady level some time following the injection. The time-course of the [Mg 2+] i change could be well accounted for by a model of longitudinal diffusion. The mean apparent Mg 2+ diffusion coefficient ( D app) was 188±9 μm 2 s −1 ( n=16), approximately four times lower than the value measured in vitro. This reduction likely results from the effects of cytoplasmic viscosity and of Mg 2+ binding to low affinity static sites. Another series of measurements was performed on fibers that were either partially or completely free of silicone: under these conditions, the time course of the change in [Mg 2+] i was in many cases more complex than predicted by simple diffusion.