Ca2+ measurement with a Ca electrode: artifacts due to some cardiotropic drugs

In order to validate D. M. Bers' method ((1982) Amer. J. Physiol. 242, C404-C408) for the determination of free [Ca] in ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid (EG-TA)-buffered Ca solution, six cardiotropic drugs were checked for their ability to interfere with Ca2+ measurements with a Ca electrode: verapamil, diltiazem, perhexiline, amrinone, bepridil, and diproteverine. Only amrinone, up to 10(-4) M, showed no effect. With the other compounds, from 5 x 10(-6) to 10(-4) M a gradual increase of the potential was observed. Negligible with 1 mM Ca2+ in the solution, the effects increased while free Ca decreased, regardless of the presence of EGTA. Similar observations were made using a K+ electrode. Relying on the results obtained by three different techniques, potentiometric study, spectrophotometric study, and binding of 45Ca on Chelex 100 resin, it was concluded that the effects of these drugs on the potential measured with an ion-specific electrode are artifacts which are not due to a release of Ca ions from the Ca-EGTA complex. When using Bers' method for the determination of free calcium, interfering drugs should be added to the solutions only after Ca2+ measurements with the Ca electrode.