Making and using calcium-selective mini- and microelectrodes

Detection and measurement of intracellular calcium concentration ([Ca(2+)](i)) have relied on various methods, the popularity of which depends on their ease of use and applicability to different cell types. Historically, Ca(2+)-selective electrodes have been used concomitantly with absorption indicators such as arsenazo-III, but their interest has been eclipsed by the introduction of a large number of fluorescent calcium probes with calcium sensitivities varying from the nanomolar to the micromolar range such as fura-2, indo-1, fluo-4, and many others. In this chapter, we emphasize the utility of Ca(2+)-selective electrodes and show that their use is complementary to use of fluorescent indicators; indeed, each method has advantages and disadvantages. We first describe the preparation and application of Ca(2+)-selective minielectrodes based on the Ca(2+) ligand ETH 129 (Schefer et al., 1986) that have a larger dynamic range and faster response time than most commercially available calcium electrodes. The second part of the chapter is dedicated to ETH 129-based Ca(2+)-selective microelectrodes (MEs), and their application in the determination of [Ca(2+)](i) in cardiac cells. Since numerous reviews and books have been dedicated to the theoretical aspects of ion-selective ME principles and technology, this chapter is not intended for investigators who have no experience with MEs.