Regulation of potassium levels by Muller cells in the vertebrate retina

The membrane properties of Muller cells, the principal glial cells of the vertebrate retina, have been characterized in a series of physiological experiments on freshly dissociated cells. In species lacking a retinal circulation (tiger salamander, rabbit, guinea pig), the end-foot of the Muller cell has a much higher K⁺ conductance than do other cell regions. In species with retinal circulation (mouse, cat, owl monkey) the K⁺ conductance of the end-foot is greater than the conductance of the proximal process of the cell. In these species, however, the K⁺ conductance of the soma and distal process is equal to, or greater than, the end-foot conductance. Muller cells also possess four voltage-dependent ion channels, including an inward rectifying K⁺ channel. These membrane specializations may aid in the regulation of extracellular K⁺ levels by Muller cells in the retina. High end-foot conductance shunts excess K⁺ out through the end-foot, where it diffuses into the vitreous humor. In vascularized retinae, excess K⁺ may also be transferred to the ablumenal wall of capillaries, where it could be transported into the blood.