Evidence against functional interaction between aquaporin-4 water channels and Kir4.1 potassium channels in retinal Müller cells.
Indirect evidence suggests that the Müller/glial cell water channel aquaporin-4 (AQP4) modulates K(+) channel function of the closely associated Kir4.1 protein. We used patch clamp to compare Kir4.1 K(+) channel function in freshly isolated Müller cells from retinas of wild-type (+/+) and AQP4 knock-out (-/-) mice. Immunocytochemistry showed a comparable Kir4.1 protein expression pattern in Müller cells from +/+ and -/- retinas, with greatest expression at their end feet. Osmotic water permeability was >4-fold reduced in -/- than in +/+ Müller cells. Resting membrane potential did not differ significantly in +/+ versus -/- Müller cells (-64 +/- 1 versus -64 +/- 1 mV, S.E., n = 24). Whole-cell K(+) currents recorded with a micropipette inserted into the cell soma were Ba(2+)-sensitive and showed no significant differences in magnitude in +/+ versus -/- Müller cells (1.3 +/- 0.1 versus 1.2 +/- 0.1 nA at -160 mV) or in inwardly rectifying current-voltage relationships. Spatially resolved K(+) currents generated by pulsed K(+) injections along Müller cell bodies were also comparable in +/+ versus -/- Müller cells. Single-channel cell-attached patch clamp showed comparable unitary conductance, current-voltage data, and open probability in +/+ versus -/- Müller cells. Thus, contrary to the generally accepted view, our results provide direct evidence against functionally significant AQP4 modulation of Müller cell Kir4.1 K(+) channel function.
Ruiz-Ederra
Javier
J
Department of Medicine, Cardiovascular Research Institute, University of California, San Francisco, CA 94143-0521, USA.
Zhang
Hua
H
Verkman
A S
AS
D000818:Animals; D051401:Aquaporin 4 / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D004594:Electrophysiology; D015353:Eye Enucleation; D007150:Immunohistochemistry; D051379:Mice; D018345:Mice, Knockout; D018408:Patch-Clamp Techniques; D010539:Permeability; D012160:Retina / Q000166:cytology / Q000502:physiology*
C496031:Aqp4 protein, mouse; D051401:Aquaporin 4