Abstract
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.
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