Changes in sodium activity during light stimulation in photoreceptors, glia and extracellular space in drone retina.

Abstract

Ion-selective micro-electrodes were used to measure Na+ activity, .alpha.Na, in the 2 types of cell, photoreceptors and glial cells and in the extracellular space, in superfused slices of the retina of the honey-bee drone, A. mellifera .male.. Movements of Na+ were induced by light stimulation, or by increasing [K+] in the superfusate. In the dark, .alpha.Na in the photoreceptors was 10 mM (S.E. of the mean = 1 mM); in the glial cells it was higher; 37 .+-. 2 mM. Apparently, in this preparation .apprx. 2/3 of the free Na+ in the tissue is in the glial cells. Stimulation with a train of light flashes, 1 s for 90 s caused .alpha.Na in the photoreceptors to increase by 16 .+-. 2 mm. K+ activity, .alpha.K, decreased by 21 .+-. 3 mM. During the standard train of light flashes, .alpha.Na in glial cells decreased by only 1.5 .+-. 0.3 mM, much less than the increase in .alpha.K (7 .+-. 2 mM). One possible interpretation of this result is that most of the increase in .alpha.K is due to K+ uptake by a mechanism other than Na+-K+ exchange. In extracellular fluid, stimulation caused .alpha.Na to fall to a relatively steady value in about 10 s. Unlike .alpha.K, there was no tendency for .alpha.Na to return to the base line during the remainder of the 90 s stimulation. The fall in .alpha.Na was 14 .+-. 1 mM: a greater fall is prevented by extracellular electric currents and a decrease in extracellular volume. When [K+] in the superfusate was increased from 7.5-18 mM, .alpha.Na decreased in the glial cells but not in the photoreceptors. In this tissue, stimulation causes changes in .alpha.Na in the neurons that might be large enough to modify the biochemistry of the cells. But in the glia, the fractional changes are small.