Evidence against functional interaction between aquaporin-4 water channels and Kir4.1 potassium channels in retinal Müller cells

Abstract

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. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.