Indocyanine Green Is A Voltage-Sensitive Fluorescent Dye

Indocyanine green (ICG) is an infrared fluorescent dye with widespread use in clinical applications, including hepatic function tests, cardiac output monitoring, and ophthalmic angiography. Using Xenopus laevis oocytes under voltage clamp, we have found that ICG is voltage-sensitive and capable of responding to changes in cellular membrane potential with a roughly linear voltage dependency. ICG's voltage response does not display the sub-millisecond kinetics characteristic of electrochromic dyes; however, it is faster than many so-called slow-response voltage probes. Tests of oocytes performed under current clamp-like conditions show that ICG can clearly follow action potentials produced at approximately 70 Hz. ICG has low expected cytotoxicity; an infrared wavelength amenable to deep tissue imaging; and the ability to label excitable cells, including cardiac myocytes and neurons. Consequently, the voltage-sensitivity of ICG presented here may have significant implications for novel applications in research and the clinic. Support: NIH GM030376.