COMPARISON BETWEEN THE SLOW CORNEA-NEGATIVE PIII COMPONENT OF THE ERGAND POTASSIUM CHANGES IN THE ISOLATED RABBIT RETINA

Light-induced extracellular potassium changes were measured in the iso lated rabbit retina superfused by a plasma saline mixture and compared with the electroretinogram. When the transmission to second-order neu rons was blocked by aspartate and glutamate or by Mg2+ the electroreti nogram consisted of the receptor potential and the cornea-negative slo w PIII. Since the onset of PIII could then be seen to precede the decr ease in extracellular potassium concentration ([K+](0)) around photore ceptors, the [K+](0) decrease could not be the cause of the onset of s low PIII. A possible source for the initial phase of slow PIII could b e the electrogenic Na+/bicarbonate transporter mechanism of glial cell s. Slow PIII depended highly on the extracellular sodium concentration , and it was larger in solutions buffered with bicarbonate than with H EPES, while the [Ki], decrease around receptors was unchanged.