Development and biological applications of chloride-sensitive fluorescent indicators

Chloride movement across cell plasma and internal membranes, is of central importance for regulation of cell volume and pH, vectorial salt movement in epithelia, and, probably, intracellular traffic. Quinolinium-based chloride-sensitive fluorescent indicators provide a new approach to study chloride transport mechanisms and regulation that is complementary to 36Cl tracer methods, intracellular microelectrodes, and patch clamp. Indicator fluorescence is quenched by chloride by a collisional mechanism with Stern-Volmer constants of up to 220 M-1. Fluorescence is quenched selectively by chloride in physiological systems and responds to changes in chloride concentration in under 1 ms. The indicators are nontoxic and can be loaded into living cells for continuous measurement of intracellular chloride concentration by single-cell fluorescence microscopy. In this review, the structure-activity relationships for chloride-sensitive fluorescent indicators are described. Methodology for measurement of chloride transport in isolated vesicle and liposome systems and in intact cells is evaluated critically by use of examples from epithelial cell physiology. Future directions for synthesis of tailored chloride-sensitive indicators and new applications of indicators for studies of transport regulation and intracellular ion gradients are proposed.