OXYGEN-CONSUMPTION IN THE RAT OUTER AND INNER RETINA - LIGHT-INDUCED AND PHARMACOLOGICALLY-INDUCED INHIBITION

Biochemical, physiological and histological data have established that 55-65% of retinal mitochondria are located in the photoreceptor inner segments and suggested that photoreceptors have at least a twofold greater oxygen consumption (QO(2)) than the remaining inner retina. QO(2) in isolated whole rat retina (Q(WR)), outer retina (Q(OR)) and inner retina (O-IR) was measured during dark and rod-saturating light adaptation. The effects of function-specific chemical agents on Q(WR), Q(OR) and Q(IR) during dark and light adaptation were determined. In addition, the oxidation-reduction (redox) potential of cytochrome a(3) of whole, outer and inner retina was measured during dark and light adaptation. During dark adaptation, the mean Q(WR) was 1.62 mu mol O-2 (mg dry wt)(-1) hr(-1) and whole retinal level of reduced cytochrome a(3) was 19%. They decreased by 24% and 37% during light adaptation, respectively. To determine Q(OR) and Q(IR) during dark and light adaptation, the outer retina was pharmaeologically-isolated from inner retina using L-2-amino-4-phosphonobutyric acid plus kynurenic acid (APB/Kyn). Experiments in the presence or absence of APB/Kyn revealed that: (i) Q(OR) but not Q(IR), of the dark-adapted retina was decreased 37% during light adaptation, (ii) the outer and inner retina consumed 65% and 35% of the Q(WR) during dark adaptation, respectively, and 54% and 46% of the O-WR during light adaptation, respectively, (iii) the level of reduced retinal cytochrome a(3) in the outer, but not inner, retina was decreased 34% during light adaptation, (iv) during light adaptation, the rate of QO(2) was equal in the outer and inner retina, and (v) the effects of APB/Kyn were reversible. These results establish that the mean rate of Q(IR) and retinal cytochrome a, are unchanged during dark or light adaptation. In addition, they suggest that Q(OR):Q(IR) in the rat may be modeled using a 65%:35% model during DA and a 55%:45% model during LA, All the function-specific agents-IBMX, lead, diltiazem, ouabain, Co2+ plus Mg2+ and verapamil-significantly decreased Q(WR) during dark and light adaptation. A more detailed analysis revealed that IBMX and lead each selectively reduced (greater than or equal to 90%) Q(OR) during dark adaptation whereas Co2+ plus Mg2+ and verapamil each selectively reduced (greater than or equal to 93%) Q(IR) during dark and light adaptation. These results are consistent with the known pharmacological sites and mechanisms of these agents. Additional experiments determined that the IBMX- and lead-induced inhibition of Q(OR) during dark adaptation resulted, either wholly or partially, from the influx of extracellular Ca2+. During dark adaptation in Ca2+-free medium: (i) Q(WR) and Q(OR) increased while Q(I)R was unchanged, (ii) Q(OR) was not decreased in the presence of IBMX and (iii) Q(OR) was only partially decreased in the presence of lead. These results are consistent with our recent findings that both Ca2+ and Pb2+ produce rapid, concentration-dependent decreases in retinal mitochondrial QO(2). Overall, these results have relevance to normal retinal function, and possible sites and mechanisms of retinal degeneration. (C) 1995 Academic Press Limited.