Magnesium ion activity in the mammalian endolymph measured with ion-selective microelectrodes

The free Mg++ concentration in endolymph was measured with Mg++-selective microelectrodes based on the neutral ligand ETH 1117. The property of Mg++ microelectrodes was obtained from calibration solutions, containing various Mg++ concentrations with the background electrolytes resembling endolymph. The range between 10 and 0.1mM Mg++ concentrations changed the potentials of Mg++ microelectrodes by 14.4±3.0 mV. The endocochlear potential and the Mg++ concentration in the endolymph were 82.0±5.0 mV and 0.77±0.29 mM in the guinea pig, and 84.4±4.9 mV and 1.12±0.24 mM in the chinchilla, respectively. These results are discussed in the light of the dependence of Na+,K+-ATPase and its interaction with Ca++.