Use of Mg2+ and Ca2+ macroelectrodes to measure binding in extracellular-like physiological solutions

Macroelectrodes designed to measure the extracellular free Mg2+ and Ca2+ concentrations ([Mg2+]o, [Ca2+]o) may be used to determine Mg2+ and Ca2+ binding to extracellular buffers. This is important, as buffer concentrations may change physiologically or experimentally. A simplified calibration method allowed [Mg2+]o and [Ca2+]o > 50 micromol/l to be accurately measured. The method was used to determine the apparent dissociation constant, Kapp (+/- SD) of Mg2+ binding to aspartate (22 degrees C, 101.7 +/- 22.5 mmol/l, n = 8; 44 degrees C, 45.2 +/- 8.3 mmol/l, n = 6), citrate (high affinity, 0.33 +/- 0.14 mmol/l, n = 4; low affinity, approximately 80 mmol/l), malate (15.9 +/- 1.0 mmol/l, n = 7) and Ca2+ binding to malate (10.3 +/- 1.1 mmol/l, n = 7). Calculated and measured Kapp for Ca2+ binding to malate were only in agreement if the concept of ionic equivalent was used to adapt the tabulated constants to the experimental conditions. For Mg2+ binding to aspartate, malate and citrate there was no or only limited agreement with the calculated Kapp. These findings emphasise the difficulties involved in calculating free concentrations in biological solutions. It is concluded that it is more accurate to measure Kapp at the appropriate ionic strength and temperature.