Consistent with this idea, there is a steep gradient of glucose concentration that declines from the RPE to the cell bodies
of photoreceptors (MacGregor et al., 1986). Second, Muller cells in the outer retina are deficient for pyruvate €
kinase, so they must have only a limited capacity to synthesize lactate (Lindsay et al., 2014). In contrast, photoreceptors
express very high levels of the M2 isoform of pyruvate kinase (Lindsay et al., 2014)