Effects of melatonin on ionic currents in cultured ocular tissues

The effects of melatonin on ionic conductances in a cultured mouse lens epithelial cell line (α-TN4) and in cultured human trabecular meshwork (HTM) cells were measured using the amphotericin perforated patch whole cell voltage-clamp technique. Melatonin stimulated a voltage-dependent Na⁺- selective current in lens epithelial cells and trabecular meshwork cells. The effects of melatonin were observed at 50 pM and were maximal at 100 μM. Melatonin enhanced activation and inactivation kinetics, but no change was observed in the voltage dependence of activation. The results are consistent with an increase in the total number of ion channels available for activation by membrane depolarization. Melatonin was also found to stimulate a K⁺- selective current at high doses (1 mM). Melatonin did not affect the inwardly rectifying K⁺ current or the delayed rectifier type K⁺ current that has been described in cultured mouse lens epithelial cells. The results show that melatonin specifically stimulated the TTX-insensitive voltage-dependent Na⁺ current by an apparently novel mechanism.