High Na+ affinity of the Na+,K+ pump in isolated rabbit retinal Müller (glial) cells

Rabbit retinal Müller (glial) cells were isolated by means of papain and mechanical dissociation. In a special perfusion chamber, the cells were penetrated with a recording microelectrode. Membrane potential changes were recorded in response to extracellular application of both high-K+ solutions and of ouabain, and that during perfusion with normal and Na+-free solutions, respectively. In other Müller cell preparations, Na+,K+-adenosine triphosphatase (ATPase) activity was measured using a radiochemical method, and its Na+ dependence was determined. All results strongly suggest that the Müller cell's Na+,K+ pump can be activated in the presence of extremely low amounts of Na+. This provides additional evidence for significant differences between the glial and the neuronal enzyme.