Increase in glial intracellular K+ in drone retina caused by photostimulation but not mediated by an increase in extracellular K+.

The predominant glial cells of the drone retina (outer pigment cells) respond to an increase in extracellular [K+] (Ko) by a net uptake of K+; thus, they contribute to bringing Ko back toward its baseline value. The authors report herein that there is also a different mechanism by which light stimulation of the retina causes an increase in intracellular free [K+] in the glial cells. In superfused retinal slices, after 5-10 minutes of continuous illumination at physiological intensities, extracellular [K+] often fell back to below its original level in the dark. This fall can be explained by increased activity of the Na/K pump in the photoreceptors and diffusion of K+ down their axons. Despite the absence of raised Ko, K+-selective microelectrodes in glial cells recorded a small increase in intracellular [K+] that was maintained for the duration of the illumination; i.e. a change occurred in the glia that was not mediated by an increase in Ko. The increase in intracellular [K+] is not mediated by illumination of the screening pigment in the glia. Unless the increase is caused by illumination of some other, unknown, pigment in the glia, the results show that some unidentified signal (that is not K+) passes from the photoreceptors to the glia.