Cyanine Dye Measurement of a Light‐Induced Transient Membrane Potential Associated with the Metarhodopsin II Intermediate in Rod‐Outer‐Segment Membranes

The fluorescent dye 3,3′-dipropylthiodicarbocyanin iodide [diS-C3−(5)] is used as a probe of fast membrane potential variations in the study of rod-outer-segment membranes. The formation of metarhodopsin II is found to be associated with the generation of a transmembrane potential, which is dissipitated by ionic movements across the membrane; its risetime is very similar to that of the protonation of the protein, which is also associated with the formation of metarhodopsin II.