Expression of a tetrodotoxin-sensitive Na+ current in cultured human retinal pigment epithelial cells

We observed a tetrodotoxin (TTX)-sensitive Na+ current in cultured fetal and adult cells of the human retinal pigment epithelium (RPE), but not in any freshly isolated fetal (n = 54) or adult (n = 47) cells, using the whole-cell version of the patch-clamp technique. A similar current was found in cultured, but not in freshly isolated, adult monkey RPE cells. The rapid activation and inactivation of this current resembled that of the voltage-dependent Na+ current of excitable cells. The voltage dependence of inactivation followed a Boltzmann function with half-maximal inactivation at -52.1 +/- 4.8 mV (n = 9), thus classifying this current as 'neuronal' in type. Recovery from inactivation followed a single exponential function with a time constant of 12.0 +/- 1.4 ms (n = 5) at -100 mV. The current was very sensitive to the Na+ channel blocker TTX, with a half-inhibition concentration of 1.87 +/- 0.37 nM (n = 5). Of special interest are the findings that current density was high when cells were rapidly proliferating and had lost their melanin pigment, and that the density declined after the cells reached confluence and repigmented. This pattern of current expression was consistently found in cells cultured with three different protocols, including a serum-free medium, indicating that serum was not necessary for its expression. We hypothesize that expression of this Na+ current in culture is regulated by an intrinsic programme related to cell differentiation. It may represent a tendency of proliferating RPE cells to dedifferentiate towards a more embryonic and neuroepithelial phenotype. Similar expression of Na+ current might occur in vivo when RPE cells proliferate, as in wounding.