Intracellular Ca indicator Quin-2 inhibits Ca²⁺ inflow via Na_i/Ca_o 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 microelectrode^1,2. 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 types^3,4. 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⁺ (Na_i)-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 Na_i-dependent Ca²⁺ influx but these same mixtures fail to block the extracellular Na⁺ (Na_O)-dependent extrusion of Ca²⁺, showing that Quin-2 acts specifically on Ca²⁺ inflow.