18 - Measurement of Calcium Flux and Intracellular Sodium by Ion-Selective Microelectrodes

Abstract

This chapter describes the measurement of calcium flux and intracellular sodium by ion-selective microelectrodes. This approach measures net transmembrane Ca 2+ movements—that is, influx–efflux, by all transport mechanisms with relatively good time resolution. This may be an advantage for comparison with assessments by a single transport mechanism, such as Ca 2+ current ( I Ca ) measurements made using voltage–clamp methods. Compared to 45 Ca 2+ flux measurements, the Ca 2+ microelectrode approach has the advantages of being on-line, having much better time resolution, and can sometimes detect smaller changes because of inherent internal controls. On the other hand, the Ca 2+ microelectrode approach does not readily allow assessment of unidirectional fluxes as can be done during 45 Ca 2+ washout experiments. Extracellular Ca 2+ transients can also be recorded using Ca 2+ -sensitive optical indicators in the interstitial space and similar results have been obtained despite vastly different experimental limitations. The chapter describes the way by which Ca 2+ -and Na + -selective microelectrodes can be tested and used in cardiac muscle. The use of ion-selective microelectrodes, both to measure extracellular and intracellular ions, can provide considerable information not only about absolute ion levels but also about transmembrane ion fluxes over relatively short time scales.