Potassium Conductance Of Cells Research Articles

A critical role for the inward rectifying potassium channel Kir7.1 in oligodendrocytes of the mouse optic nerve

Inward rectifying potassium channels (Kir) are a large family of ion channels that play key roles in ion homeostasis in oligodendrocytes, the myelinating cells of the central nervous system (CNS). Prominent expression of Kir4.1 has been indicated in oligodendrocytes, but the extent of expression of other Kir subtypes is unclear. Here, we used qRT-PCR to determine expression of Kir channel transcripts in the mouse optic nerve, a white matter tract comprising myelinated axons and the glia that support them. A novel finding was the high relative expression of Kir7.1, comparable to that of Kir4.1, the main glial Kir channel. Significantly, Kir7.1 immunofluorescence labelling in optic nerve sections and in isolated cells was localised to oligodendrocyte somata. Kir7.1 are known as a K+ transporting channels and, using patch clamp electrophysiology and the Kir7.1 blocker VU590, we demonstrated Kir7.1 channels carry a significant proportion of the whole cell potassium conductance in oligodendrocytes isolated from mouse optic nerves. Notably, oligodendrocytes are highly susceptible to ischemia/hypoxia and this is due at least in part to disruption of ion homeostasis. A key finding of this study is that blockade of Kir7.1 with VU590 compromised oligodendrocyte cell integrity and compounds oligodendroglial loss in ischemia/hypoxia in the oxygen–glucose deprivation (OGD) model in isolated intact optic nerves. These data reveal Kir7.1 channels are molecularly and functionally expressed in oligodendrocytes and play an important role in determining oligodendrocyte survival and myelin integrity.

Voltage dependence of the basolateral membrane conductance in the Amphiuma collecting tubule.

The basolateral potassium conductance of cells of most epithelial cells plays an important role in the transcellular sodium transport inasmuch as the large negative equilibrium potential of potassium across this membrane contributes to the electrical driving force for Na+ across the apical membrane. In the present study, we have attempted to establish the I-V curve of the basolateral membrane of the Amphiuma collecting tubule, a membrane shown to be K+ selective. Transepithelial I-V curves were obtained in short, isolated perfused collecting tubule segments. The "shunt" conductance was determined using amiloride to block the apical membrane Na+ conductance. In symmetrical solutions, the "shunt" I-V curve was linear (conductance: 2.2 +/- 0.3 mS.cm-2). Transcellular current was calculated by subtracting the "shunt" current from the transepithelial current in the absence of amiloride. Using intracellular microelectrodes, it was then possible to measure the basolateral membrane potential simultaneously with the transcellular current. The basolateral conductance was found to be voltage dependent, being activated by hyperpolarization: conductance values at -30 and -80 mV were 3.6 +/- 1.0 and 6.6 +/- 1.0 mS.cm-2, respectively. Basolateral I-V curves were thus clearly different from that predicted by the "constant field" model. These results indicate that the K+-selective basolateral conductance of an amphibian collecting tubule shows inward ("anomalous") rectification. Considering the electrogenic nature basolateral Na-K-pump, this may account for coupling between pump-generated potential and basolateral K+ conductance.

A quantitative analysis of glial cell coupling in the retina of the axolotl ( Ambystoma mexicanum)

The strength of gap junctional coupling of radial glial cells (Mu¨ller cells) in the isolated axolotl retina was assessed by monitoring the spread of dye between cells, and by injecting current into one cell and recording the voltage response in surrounding cells. Dye injected into one Mu¨ller cell spread to surrounding Mu¨ller cells, and could be detected up to 130 μm away, i.e. over 4 times the mean Mu¨ller cell spacing of 30 μm. Injecting 1 nA of current into a Mu¨ller cell evoked responses of 7 mV in that cell, 1 mV in next neighbour cells, and 0.2 mV in cells at 60 μm distance. Analysis of these data indicates an electrical space constant for the Mu¨ller cell network of 15 μm, and predicts that isolated cells should have a resistance of 11.4 MΩ. Mu¨ller cells isolated by papain dissociation of the retina were found, by whole-cell patch-clamping, to have a mean resistance of 12.4 MΩ. These results on lateral coupling are combined with data showing that over 90% of the Mu¨ller cell potassium conductance is in the vitreal endfoot of these cells to provide a fairly complete electrical description of the radial glial cell network in the retina. Gap junctional coupling of Mu¨ller cells increases by 60% the ‘spatial buffering’ that these glial cells can carry out to reduce localized rises in extracellular potassium concentration. The location of the majority of the Mu¨ller cell potassium conductance in the cell endfoot ensures that laterally buffered K + is deposited in the vitreous, rather than depolarizing surrounding retinal neurones.