Voltage-dependent sodium channels develop in rat retinal pigment epithelium cells in culture

Whole-cell patch-clamp recordings were made from isolated cells of the retinal pigment epithelium (RPE) of neonatal rats. After 24 hr in cell culture, RPE cells developed a transient, voltage-activated inward current that was never observed in acutely isolated cells. The kinetics, voltage-dependence, and reversal potential of the current and its dependence on external sodium demonstrated that the current was due to the expression of voltage-activated Na+ channels in cultured RPE cells. The current was partly blocked by tetrodotoxin at low concentrations (< 100 nM), but a second component of Na+ current was unblocked by tetrodotoxin at concentrations up to 10 microM. Na+ channels were present in cultured RPE cells at sufficient density to support regenerative action potentials in voltage recordings. Both the epithelial cells of the RPE and the neurons of the retina derive embryonically from neural origin, and it is known that under certain circumstances, nonmammalian RPE cells retain the ability to take on neuronal characteristics. The development of voltage-activated Na+ channels and the presence of action potentials demonstrate that neonatal mammalian RPE cells are also capable of expressing neuronal characteristics in cell culture.