Na+ currents through Ca2+ channels in human retinal glial (Muller) cells

Purpose. To detect the presence of voltage-gated Ca2+ channels in the plasm a membranes of freshly isolated Muller glial cells from the human retina an d their modulation by GABA(B) receptor agonists. Methods. Whole cell voltage-clamp recordings were made to study Ca2+, Ba2+, and Na+ currents through voltage-gated Ca2+ channels. Results. The vast majority of the investigated cells displayed no resolvabl e currents through Ca2+ channels when Ca2+ ions (2 mM) were present in the extracellular solution. Small-amplitude inwardly directed currents (similar to 0.6 pA/pF) were detected when Ba2+ ions (20 mM) were used as charge car rier. However, when Na+ ions were used as charge carrier in divalent cation -free external solution, currents of large amplitudes (similar to 7.5 pA/pF ) through voltage-gated Ca2+ channels were detected. Human Muller cells dis played currents through both transient, low voltage-activated Ca2+ channels and long-lasting, high voltage-activated channels. The Na+ fluxes through low voltage-activated Ca2+ channels were inhibited in a voltage-independent manner in the presence of GABA(B) receptor agonists. Conclusions. Human Muller glial cells express different kinds of voltage-ga ted Ca2+ channels in their plasma membranes that can be activated only unde r certain physiological or pathophysiological conditions. The record of Na fluxes in divalent cation-free solutions may be a technique to detect the presence of "hidden" voltage-gated Ca2+ channels in Muller glial cells.