CALIBRATION OF MG2-SELECTIVE MACROELECTRODES DOWN TO 1 MU-MOL 1(-1) IN INTRACELLULAR AND CA2+-CONTAINING EXTRACELLULAR SOLUTIONS()

Using Mg2+-selective macroelectrodes based on the neutral carriers ETH 7025 and ETH 5506, methods were developed to determine accurately the apparent binding constant (K-app) and purity, and hence the ionized m agnesium concentration ([Mg2+]), in Ca2+-free, Mg2+ buffer solutions m anufactured with either CDTA trans-1,2-diaminocyclohexane-N,N,N',N'-te traacetic acid monohydrate) or EDTA. In nominally Ca2+-free solutions, calibration of the macroelectrodes was possible down to 1 mu mol l(-1 ) in both intracellular (ETH 7025)- and extracellular (ETH 5506)-like physiological solutions. The measured [Mg2+] in the buffer solutions o verlapped with the [Mg2+] set by dilution alone, suggesting that the m ethod was reliable. These buffer solutions could then be used to manuf acture standard Mg2+ solutions containing known [Mg2+] at a set calciu m concentration. Ca2+ sensitivity limits the use of ETH 7025 and, for extracellular measurements in Ca2+-containing solutions, Mg2+-selectiv e macroelectrodes manufactured with ETH 5506 were the electrodes of ch oice. In 0.5-0.9 mmol l(-1) Ca+, measurement of [Mg2+] was possible do wn to 10 mu mol l(-1). At [Mg2+] greater than 0.25 mmol l(-1) there wa s no interference from Ca2+ (0.5-1.5 mmol l(-1)). With CDTA and/or EDT A buffer solutions there was a wide variation between the calculated v alues for the [Mg2+] and the measured [Mg2+] (the calculated values di ffered by a factor of up to 4.5 and 3, respectively). At present, meas urement of the K-app and ligand purity in the appropriate solution at the desired pH and temperature would seem to be the best strategy to a dopt rather than attempting to calculate the constant. Since no recogn ized international standard exists for [Mg2+] at the micromolar level, values in the literature for K-d, etc. in this range can only be rega rded as approximate.