Ethylrhodamine as a fluorescent penetrating cation and a membrane potential-sensitive probe in cyanobacterial cells

Abstract

Ethylrhodamine, a homolog of rhodamine 123, was found to generate a diffusion potential across a planar bilayer phospholipid membrane (the compartment with the lower ethylrhodamine concentration positive). Addition of a penetrating synthetic anion increased the sensitivity of the membrane to ethylrhodamine, an effect similar to that previously described for triphenylmethylammonium, tetraphenylphosphonium and other synthetic penetrating cations. In experiments with the filamentous multicellular cyanobacteria Phormidium uncinatum, ethylrhodamine was used as a cationic penetrant and vital fluorescent probe for membrane potential (δΨ). It is shown that a treatment increasing protonic potential (illumination) and converting δpH to δΨ (nigericin) enhanced the fluorescence of cyanobacteria, as ethylrhodamine accumulated inside the cells. Uncouplers discharging δΨ completely abolished the fluorescence increase. Individual variations in the fluorescence of cells composing cyanobacterial trichomes could be observed. Illumination of a small part of a trichome with a narrow beam of the light actuating photosynthesis was shown to induce a fluorescence increase in both illuminated and non-illuminated regions of this trichome, indicating δΨ transmission from cell to cell.