Potassium activity in photoreceptors, glial cells and extracellular space in the drone retina: changes during photostimulation.

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Abstract
A double-barrelled potassium-sensitive micro-electrode was developed that was fine enough to record intracellular electrical potentials and K activities (.alpha.K) in the drone [Apis mellifera] retina. .alpha.K was measured in the photoreceptor cells, in the pigment (glial) cells, and in the extracellular space, in the superfused, cut, retina. The effect of photostimulation was studied: 20 ms light flashes, intense enough to evoke receptor potentials of maximum amplitude were presented, 1/s, in a train lasting about 2 min. In photoreceptors with membrane potentials .gtoreq. 50 mV, .alpha.K in the dark was 79 mM, S.D. = 27 mM, n = 11. During photostimulation .alpha.K fell by 21.5 .+-. 9.5 mM with a half-time of 30 .+-. 22 s. In pigment cells with membrane potentials .gtoreq. 50 mV, .alpha.K in the dark was 52 mM, SD = 13 mM, n = 11. During photostimulation .alpha.K increased by 14 .+-. 5 mM. In the extracellular space .alpha.K increased during photostimulation with a mean half-time of < to a maximum (mean value 14 mM, SD = 8.4 mM, n = 22), and then fell to a plateau. It is estimated from the anatomy that the photoreceptors occupy approximately 38% of the total volume of the retina, the pigment cells 57%, and the extracellular space 5%. It seems possible that during photostimulation nearly all the net loss of K from the photoreceptors is temporarily stored in the pigment cells. Recordings were made in the extracellular space of the intact animal by passing the electrode through a hole in the cornea. The mean .alpha.K in the dark was 7.7 mM, SE = 0.4 mM, n = 22. In the intact animal the amplitude and time course of the change in extracellular .alpha.K evoked by the standard pattern of photostimulation were within the range observed in the superfused preparation. For most of the K-sensitive micro-electrodes an ion-exchanger based on potassium tetra-p-chlorophenylborate was used. This ion-exchanger is sensitive to acetylcholine, so further recordings were made in extracellular space with microelectrodes based on valinomycin which were insensitive to acetylcholine. The measurements of .alpha.K were not noticeably contaminated by a response to acetylcholine. When the glial cells take up K during photostimulation they simultaneously lose Na. These ion movements can be summarized as tending to maintain homeostasis of .alpha.K and .alpha.Na in extracellular space.