Introduction: importance of Mg in physiology and medicine and the need for ion selective electrodes.

Mg is a co-factor in more than 325 enzyme systems in cells and is the second most abundant intracellular cation after K. It is important in numerous physiologic and homeostatic functions such as hormone-receptor binding, gating of Ca2+ channels, transmembrane flux of ions, regulation of adenylate cyclase, Ca(2+)-Ca(2+) release, muscle contraction, cardiac excitability, neuronal activity, control of vasomotor tone and neurotransmitter release, to name a few. Serum total Mg measurements suggests this extracellular cation is often lowered in many hospitalized patients. It appears to be greatly depleted in normal food preparation and processing. Its daily intake has been declining steadily in the USA from where it was about 500 mg/day, at the turn of the century, to where it is now about 175-225 mg/day. Most mammals exhibit very similar levels of extracellular Mg, including humans. This might suggest that total Mg measurements on serum or plasma aren't sensitive blood variables or analytes to reflect what may be going on in the body at the cellular level. Mg exists in several forms in the blood, but the ionized, free Mg is probably the most physiologically important fraction, thus pointing to a need for sensitive, specific ion selective electrodes. Our studies, so far, suggest that ISE's can be designed which will work equally well on whole blood, serum and plasma to measure IMg2+ in healthy and diseased subjects.