Ca(2+)-conductances in cultured rat retinal pigment epithelial cells

Membrane conductances for Ca2+ in cultured rat pigment epithelial cells were studied in the whole-cell configuration of the patch-clamp technique using barium (10 mM) as a charge carrier. Two types of voltage-dependent and verapamil- and diltiazem-sensitive Ba2+ currents were observed. First, a nearly sustained current was activated by depolarization to potentials more positive than -30 mV and blocked by nifedipine (1 microM). This current was observed in cells of primary cultures less than 13 days old. Second, a transient nifedipine (1 microM) insensitive current was activated by depolarization to potentials more positive than -55mV in cultures which were more than 13 days old. This current was not carried by sodium and blocked by 1 microM tetrodotoxin (TTX). In summary, cultured rat retinal pigment epithelial cells in younger primary cultures express Ba2+ currents indicating the presence of L-type Ca2+ channels. In older primary cultures a low-voltage activated channel was observed with properties different from T-type calcium channels or TTX-sensitive calcium conducting sodium channels.