Methods for the measurement of membrane potentials in cells not easily penetrated by microelectrodes were assessed for use in epithelia. The lipophilic cation triphenylmethylphosphonium appeared to distribute in a Nernstian fashion in the epithelial cells of Necturus gallbladder as judged by parallel microelectrode measurements. In the toad urinary bladder, the distribution of this cation gave a value for the membrane potential of epithelial cells under short-circuit conditions of -62 mV in normal Ringer's solution and -51 mV after 2 h treatment with ouabain. In our laboratory the dye 3,3'-dipropylthiadicarbocyanine iodide, when used with cell suspensions, yielded results comparable to those of other workers, but we were unable to record a redistributional signal from epithelial sheets or scraped cells. The dye appeared to enter cells and become irreversibly bound. The membrane-bound merocyanine dyes have not been used on epithelial cells. They appear to hold the greatest promise for dynamic experiments on epithelial membrane potentials.
