Effects of photoreceptor metabolism on interstitial and glial cell pH in bee retina: evidence of a role for NH4

1. Measurements were made with pH microelectrodes in superfused slices of the retina of the honey-bee drone. In the dark, the mean +/- S.E.M. pH values in the three compartments of the tissue were: neurones (photoreceptors), 6.99 +/- 0.04; glial cells (outer pigment cells), 7.31 +/- 0.03; extracellular space, 6.60 +/- 0.03. 2. Stimulation of the photoreceptors with light caused transient pH changes: a decrease in the photoreceptors (pHn) and in the glial cells (pHg), and an increase in the interstitial clefts (pHo). 3. The effects of inhibition and activation of aerobic metabolism showed that part, perhaps all, of the light-induced delta pHo resulted from the increased aerobic metabolism in the photoreceptors. 4. Addition of 2 mM NH4+ to the superfusate produced changes in pHo and pHg of the same sign as and similar amplitude to those caused by light stimulation. Manipulation of transmembrane pH gradients had similar effects on changes in pHo induced by light or by exogenous NH4+. 5. Measurements with NH(4+)-sensitive microelectrodes showed that stimulation of aerobic metabolism in the photoreceptors increased [NH4+]o and also that exogenous NH4+/NH3 was taken up by cells, presumably the glial cells. 6. We conclude that within seconds of an increase in the aerobic metabolism in the photoreceptors, they release an increased amount of NH4+/NH3 which affects pHo and enters glial cells. Other evidence suggests that in drone retina the glial cells supply the neurones with amino acids as substrates of energy metabolism; the present results suggest that fixed nitrogen is returned to the glial cells as NH4+/NH3.