The time course of the light-induced extracellular potassium change around receptors and at the vitreal surface compared with the time course of slow PIII wave in the isolated rabbit retina.

The experimental preparation was a piece of isolated rabbit retina, the lower side of which was superfused in a chamber with a plasma-saline mixture kept at 35 degrees C to which, in some experiments, either sodium aspartate and glutamate, or Mg2+, were added to suppress second order neuronal activity. The upper side was exposed to an atmosphere of humidified oxygen. With longlasting light stimuli (30-180 s) the [K+]0 around receptors first falls from its dark-adaptation value (4.6 +/- 0.4 mM) to 3.3 +/- 0.6 mM and then reaccumulates nearly to the pre-stimulus value. The time course of reaccumulation varies greatly (30-180 s) between preparations. The peak time (90% value) of the [K+]0 decrease is several seconds. It seems that the more rapidly the retina is prepared the shorter the peak time of the [K+]0 reaccumulation and the more rapid the [K+]0 reaccumulation. This suggests that the [K+]0 reaccumulation is effected by active, oxygen-dependent mechanisms. PIII always precedes the [K+]0 change, its peak time is several 100 ms. During longlasting light stimuli (30-180 s) slow PIII reflects the difference in the [K+]0 changes around the photoreceptors and at the vitreal surface. The rapidly and carefully isolated preparations exhibit a slow cornea-positive potential similar to a c-wave, when second order neuronal activity is not suppressed.