Dicarbocyanine fluorescent probes of membrane potential block lymphocyte capping, deplete cellular ATP and inhibit respiration of isolated mitochondria.

3,3'-Dipropylthiodicarbocyanine iodide, a widely used fluorescent probe of membrane potential, was found to inhibit anti-Ig antibody, induced capping of mouse lymphocytes. The dye also lowered the cell ATP content. Experiments with isolated mitochondria revealed that the probe had a potent inhibitory action at site I of the respiratory chain. This mitochondrial blockade helps to explain the ATP depletion and blockade of capping, and gives cause for caution in the use of this dye as a probe of cell membrane potential. Three related dicarbocyanine dyes had similar toxic effects, but two cyanine dyes with much longer alkyl side chains, which have been used as probes of membrane fluidity, did not.