Differential Effect of Ischemia on Ionic Homeostasis in the Inner and Outer Mammalian Retina

Abstract

Abstract: : Purpose: The retina exhibits a regional sensitivity to ischemia, with the inner layers more sensitive than the outer layers. We hypothesized that this differential effect of ischemia might be associated in part with the specific membrane characteristics and ionic fluxes of the photoreceptors and ganglion cells (GCs). We have therefore studied how ischemia alters ionic homeostasis in the inner and outer retina. Methods: Ion–selective microelectrodes were used to measure extracellular ionic concentrations in the rabbit eyecup preparation. Patch–clamp recordings in the whole–cell configuration were used to record ischemia–dependent changes in the membrane potential of individual GCs, and changes in intracellular free Ca²⁺ in the same cells were simultaneously measured using Ca–imaging. In our model of experimental ischemia, standard oxygen–glucose deprivation was enhanced by replacing NaHCO₃ with NaCl. Results:Experimental ischemia evoked an increase in extracellular K⁺ and a decrease in extracellular Ca²⁺, but these effects were different in different retinal regions. In the ganglion cell layer (GCL) the changes were biphasic with slow and relatively small initial changes and faster and larger changes in a second phase that developed 15–20 min after the onset of ischemia. In the photoreceptor layer the ionic changes were single–phased and significantly smaller than in the GCL. Recordings from individual GCs showed that ischemia produced a two–phased depolarization that was similar in time course to the ionic changes in the GCL. Intracellular Ca²⁺ did not increase at the onset of ischemia, as might be expected, but instead increased 15 – 20 min later, in a manner that was coincident with the second fast phase of changes that occurred in ionic homeostasis in the GCL and in the membrane potential of GCs. Conclusions:The data show that ischemia affects the ionic homeostasis of GCs differently than that of photoreceptor cells. We suggest that the reason for this is the distinct difference in membrane characteristics of these two types of retinal neuron. Specifically, GCs possess Na⁺ voltage–gated and ionotropic glutamate–gated channels that photoreceptors lack. As a result, the ischemia–induced cellular depolarization and resultant increase in intracellular Ca²⁺ occur to a much greater extent in GCs, compared to photoreceptor cells. These differences in voltage and neurotransmitter–gated channels may contribute to the greater ischemia–induced damage that occurs in the inner, compared to the outer retina.

Keywords: ischemia • excitatory neurotransmitters • electrophysiology: non-clinical