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

Abstract

Purpose. To detect the presence of voltage-gated Ca 2+ channels in the plasma membranes of freshly isolated Müller glial cells from the human retina and their modulation by GABA B receptor agonists. Methods. Whole cell voltage-clamp recordings were made to study Ca 2+, Ba 2+, and Na + currents through voltage-gated Ca 2+ channels. Results. The vast majority of the investigated cells displayed no resolvable currents through Ca 2+ channels when Ca 2+ ions (2 mM) were present in the extracellular solution. Small-amplitude inwardly directed currents (~0.6 pA/pF) were detected when Ba 2+ ions (20 mM) were used as charge carrier. However, when Na + ions were used as charge carrier in divalent cation-free external solution, currents of large amplitudes (~7.5 pA/pF) through voltage-gated Ca 2+ channels were detected. Human Müller cells displayed currents through both transient, low voltage-activated Ca 2+ channels and long-lasting, high voltage-activated channels. The Na + fluxes through low voltage-activated Ca 2+ channels were inhibited in a voltage-independent manner in the presence of GABA B receptor agonists. Conclusions. Human Müller glial cells express different kinds of voltage-gated Ca 2+ channels in their plasma membranes that can be activated only under 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 Ca 2+ channels in Müller glial cells.