CALCIUM CONTENT OF THE ERYTHROCYTES: A SENSITIVE AND EASY HANDLING METHOD FOR MEASURING FREE CALCIUM IONS, AND MODULATION OF THE Ca2+ ION CONCENTRATION BY THE CALCIUM ANTAGONISTS NIFEDIPINE AND PENTOXIFYLLINE

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Abstract
1. A method for determining free Ca2+-ions in the erythrocyte is described, using a commercially available ORION-Ca-electrode and calomel reference electrode assembly, where changes in free Ca2+-ion concentration upon addition of 0.01% digitonin could be measured. 2. The average value found for fresh cells from 20 healthy donors at 37°C (pH = 7.4) was 0.20 ± 0.04 μmol/L referred to a haematocrit of 10%. 3. Decrease of the simultaneously determined adenosinetriphosphate (ATP) concentration indicates that ATP is presumably needed to activate the Ca-ATPase. 4. In vitro addition of the calcium antagonists pentoxifylline and nifidepine, respectively, induced a normalization of the intraerythrocytic Ca2+-ion concentration after previous increase with the ion carrier ionophore A23187. 5. The advantages and possible clinical applications of this method are discussed