Na(+) currents through Ca(2+) channels in human retinal glial (Müller) cells.
OBJECTIVE
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 ( approximately 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 ( approximately 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.
Bringmann
A
A
Department of Neurophysiology, University of Leipzig, Paul Flechsig Institute of Brain Research, Leipzig, Germany. bria@server3.medizin.uni-leipzig.de
Biedermann
B
B
Faude
F
F
Enzmann
V
V
Reichenbach
A
A
D001464:Barium / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism*; D002462:Cell Membrane / Q000502:physiology; D004594:Electrophysiology; D006801:Humans; D008564:Membrane Potentials; D009457:Neuroglia / Q000502:physiology*; D018080:Receptors, GABA-B / Q000378:metabolism; D012160:Retina / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*
D015220:Calcium Channels; D018080:Receptors, GABA-B; D001464:Barium; D012964:Sodium; D002118:Calcium