||"The high Mg content of mammalian ventricular muscle is an old observation (ref 6). The Mg content of plasma and interstitial space is relatively small. Most of the Mg is therefore inside myocardial cells. It has recently become possible to correct for extracellular Mg in rat ventricular muscle with increased accuracy, because more reliable measurements of extracellular volume can now be made (ref 7). The cellular Mg content of rat ventricular muscle obtained with the help of such a measurement is 43.4±0.4 mmoles/kg dry weight, a value that is in good agreement with previous estimates. The calculated intracellular Mg concentration is 17.3±0.2 mmoles/kg cell water, a value that is roughly comparable to that of 16.1mM for rat skeletal muscle (primary source)."