Intracellular Ca indicator Quin-2 inhibits Ca2+ inflow via Nai/Cao exchange in squid axon

Abstract

Until recently, intracellular free calcium has been amenable to measurement and investigation only in cells large enough to permit either microinjection of a suitable Ca sensor such as aequorin or arsenazo III or insertion of a Ca-sensitive microelectrode1,2. This constraint on cell size was removed by the development of the fluorescent Ca2+-sensitive dye Quin-2 and its acetoxymethyl ester, which can be introduced into a wide range of cell types3,4. A major requirement of any intracellular Ca2+ indicator is that it should not disturb intracellular Ca2+ homeostasis and Quin-2 is generally considered to be satisfactory in this respect5. We now report that injection of Quin-2 into squid (Loligo forbesi) axons can almost completely abolish one component of Ca2+ entry—intracellular Na+ (Nai)-dependent Ca2+ inflow, which occurs via Na/Ca exchange6. Mixtures of Ca and Quin-2 that buffer an ionized Ca2+ at close to physiological concentrations also block Nai-dependent Ca2+ influx but these same mixtures fail to block the extracellular Na+ (NaO)-dependent extrusion of Ca2+, showing that Quin-2 acts specifically on Ca2+ inflow.

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Allen, T., Baker, P. Intracellular Ca indicator Quin-2 inhibits Ca2+ inflow via Nai/Cao exchange in squid axon. Nature 315, 755–756 (1985). https://doi.org/10.1038/315755a0

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