Ca(2+) channel-mediated currents in retinal glial (Müller) cells of the toad (Bufo marinus)

Whole-cell voltage-clamp recordings were used to detect voltage-gated Ca(2+) channels in freshly isolated retinal glial (Müller) cells of the toad (Bufo marinus). Using Ca(2+) ions (2 mM) as charge carriers (in the presence of 1 mM Mg(2+)), no inwardly directed currents could be observed during the application of depolarizing voltage steps. However, after omitting the divalent cations from the bath solution, large-amplitude inwardly directed currents were evoked that were carried by Na(+) ions, and were mediated by at least two different kinds of Ca(2+) channels, transient low voltage-activated (LVA) channels and sustained high voltage-activated (HVA) channels. While the LVA currents activated at potentials positive to -90 mV and peaked at -40 mV, the HVA currents activated positive to -60 mV and peaked at -20 mV. It is concluded that Müller glial cells of the toad express distinct types of voltage-gated Ca(2+) channels that may be activated, under certain conditions, close to physiological membrane potentials.