Intracellular Mg2+ diffusion within isolated rat skeletal muscle fibers

Intracellular free magnesium concentration ([Mg2+](i)) was measured in enzy matically isolated rat skeletal muscle fibers using the fluorescent dye mag -indo-1. The change in [Mg2+](i) produced by a local intracellular microinj ection of magnesium pidolate (magnesium pyrrolidone carboxylate) was measur ed at a given distance from the injection site. In one series of experiment s this protocol was tested on isolated fibers that were completely embedded into silicone grease: under these conditions, the injection produced an in crease in [Mg2+](i) that reached a steady level some time following the inj ection. The time-course of the [Mg2+](i) change could be well accounted for by a model of longitudinal diffusion. The mean apparent Mg2+ diffusion coe fficient (D-app) was 188 +/- 9 mum(2) s(-1) (n = 16), approximately four ti mes lower than the value measured in vitro. This reduction likely results f rom the effects of cytoplasmic viscosity and of Mg2+ binding to low affinit y static sites. Another series of measurements was performed on fibers that were either partially or completely free of silicone: under these conditio ns, the time course of the change in [Mg2+](i) was in many cases more compl ex than predicted by simple diffusion. (C) 2001 Elsevier Science B.V. All r ights reserved.