The concentrations of free magnesium and free calcium ions both increase in skeletal muscle fibres entering Rigor mortis.

Metabolic inhibitors have been used to induce rigor in mammalian muscle fibres previously injected with the Ca(2+)- and Mg(2+)-binding dye arsenazo III. The spectral changes which ensue, after the onset of rigor, indicate that a large increase in intracellular free Ca(2+) occurs (to more than 10(-4)m) but that this is preceded and accompanied by a substantial increase in free Mg(2+) concentration (to 2 mm or more) presumably as a result of the decline in the ATP concentration within the cell. Fibres in rigor have been treated with the divalent cation ionophore A23187 to enable Ca(2+) and Mg(2+) to be extracted selectively from the cell by extracelluar chelating agents. Optical measurements performed during this extraction, confirm that both Ca(2+)- and Mg(2+)-binding to arsenazo III contribute to the absorption change which is recorded, whilst the treatment of relaxed fibres with the ionophore together with EDTA, produces an absorption change which, in combination with other measurements, indicates that in the sarcoplasm of the resting relaxed cell, approximately one third of the dye is complexed with Mg(2+). This indicates that the cytoplasmic free Mg(2+) concentration, prior to rigor onset, is about 0·5 mm.