Kir potassium channel subunit expression in retinal glial cells: Implications for spatial potassium buffering

To understand the role of different K+ channel subtypes in glial cell-mediated spatial buffering of extracellular K+, immunohistochemical localization of inwardly rectifying K+ channel subunits (Kir2.1, Kir2.2, Kir2.3, Kir4.1, and Kir5.1) was performed in the retina of the mouse. Stainings were found for the weakly inward-rectifying K+ channel subunit Kir4.1 and for the strongly inward-rectifying K+ channel subunit Kir2.1. The most prominent labeling of the Kir4.1 protein was found in the endfoot membranes of Muller glial cells facing the vitreous body and surrounding retinal blood vessels. Discrete punctate label was observed throughout all retinal layers and at the outer limiting membrane. By contrast, Kir2.1 immunoreactivity was located predominantly in the membrane domains of Miller cells that contact retinal neurons, i.e., along the two stem processes, over the soma, and in the side branches extending into the synaptic layers. The results suggest a model in which the glial cell-mediated transport of extracellular K+ away from excited neurons is mediated by the cooperation of different Kir channel subtypes. Weakly rectifying Kir channels (Kir4.1) are expressed predominantly in membrane domains where K+ currents leave the glial cells and enter extracellular "sinks," whereas K+ influxes from neuronal "sources" into glial cells are mediated mainly by strongly rectifying Kir channels (Kir 2.1). The expression of strongly rectifying Kir channels along the "cables" for spatial buffering currents may prevent an unwarranted outward leak of K+, and, thus, avoid disturbances of neuronal information processing.