Comparison between the slow cornea-negative PIII component of the ERG and potassium changes in the isolated rabbit retina.

Light-induced extracellular potassium changes were measured in the isolated rabbit retina superfused by a plasma saline mixture and compared with the electroretinogram. When the transmission to second-order neurons was blocked by aspartate and glutamate or by Mg2+, the electroretinogram consisted of the receptor potential and the cornea-negative slow PIII. Since the onset of PIII could then be seen to precede the decrease in extracellular potassium concentration ([K+]0) around photoreceptors, the [K+]0 decrease could not be the cause of the onset of slow PIII. A possible source for the initial phase of slow PIII could be the electrogenic Na+/bicarbonate transporter mechanism of glial cells. Slow PIII depended highly on the extracellular sodium concentration, and it was larger in solutions buffered with bicarbonate than with HEPES, while the [K+]0 decrease around receptors was unchanged.