Light-induced pH changes in the intact retinae of normal and early diabetic rats

•Light-induced pH changes in the intact rat retina were measured by microelectrodes.•In the retina, pH changes were not much different in control and early diabetic rats.•In the choroid, pH changes were different in control and early diabetic rats.•Evacuation of retinal H+ by the choroid appears to be compromised in early diabetics.

Double-barreled H+-selective microelectrodes were used to measure local extracellular concentration of H+ ([H+]o) in the retina of dark-adapted anesthetized Long-Evans rats. The microelectrode advanced in steps of 30 μm throughout the retina from the vitreal surface to retinal pigment epithelium and then to the choroid, recording changes in [H+]o evoked by light stimulation. Recordings were performed in diabetic rats 1-3 months after intraperitoneal injection of streptozotocin and the results were compared with data obtained in age-matched control animals. Brief light stimulation (2.5 s) evoked changes of [H+]o with amplitudes of a few nM. Throughout the retina, there was a transient initial acidification for ∼200 ms followed by steady alkalinization, although amplitudes and kinetics of these components were slightly variable in different retinal layers. No significant difference was found when the light-induced [H+]o changes recorded in various retinal layers of early diabetic rats were compared with the [H+]o changes from corresponding layers of control animals. Also, when H+-selective microelectrodes were located in the retinal pigment epithelium (RPE) layer, an increase in H+ was recorded, whose time course and amplitude were similar in control and diabetic rats. However, a striking difference between light-induced [H+]o changes in controls and diabetics was observed in the choriocapillaris, in the thin layer (10-20 μm) distal to the basal membrane of the RPE. In control rats, choroidal [H+]o decreased in a few cases, but much more often practically did not change. In contrast, diabetic rats demonstrated either an increase (in half of the cases) or no change in choroidal [H+]o. The data suggest that the active participation of the choroidal blood supply in stabilization of [H+]o could be partially compromised already at early stages of diabetes in rats. Interestingly, it appeared that the acid removal by the choroidal circulation was compromised most after 1 month of diabetes and tended to improve later.