Intracellular ATP activates inwardly rectifying K+ channels in human and monkey retinal Müller (glial) cells.

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Abstract
1. In the vertebrate retina, the inwardly rectifying K+ (KIR) channels of the Müller (glial) cells are pathways for the redistribution of excess extracellular K+. Due to this role in K+ homeostasis, the activity of Müller cell KIR channels is likely to have significant functional consequences for the retina. In this study we asked whether intracellular ATP regulates the function of KIR channels expressed by Müller cells, the principal glia of the retina. 2. Freshly dissociated Müller cells from the human and monkey (Macaca fascicularis) retina were studied with various configurations of the patch‐clamp technique. 3. Whole‐cell recordings from Müller cells revealed that a run‐down of the inwardly rectifying K+ current (IK(IR)) was prevented if the pipette solution contained Mg‐ATP. Chemical ischaemia induced by inhibitors of glycolysis and oxidative phosphorylation caused a nearly 10‐fold reduction in the IK(IR)) that was fully restored when metabolically inhibited Müller cells were internally perfused with ATP. 4. In recordings from membrane patches of fresh primate Müller cells, we found that inward‐rectifying channels with a conductance of 20 pS in 100 mM Ko+ were the predominant type of KIR channel. In excised patches these 20 pS KIR channels were activated when Mg‐ATP was at the cytoplasmic surface. Experiments with inside‐out patches indicated that the activity of the 20 pS KIR channels can be maintained by ATP synthesized at sites located close to the channel. 5. The inability of the non‐hydrolysable ATP analogue 5'‐adenylylimidodiphosphate (AMP‐PNP) to prevent the run‐down of IK(IR))and the Mg2+ dependence of the ATP effect on KIR channels are consistent with a mechanism of activation requiring the hydrolysis of ATP. 6. These observations suggest that the metabolic state of a Müller cell regulates the activity of its 20 pS KIR channels and thus influences the function of the glial cell in maintaining K+ homeostasis in the retina.