AN ASSESSMENT OF RAT PHOTORECEPTOR SENSITIVITY TO MITOCHONDRIAL BLOCKADE

Purpose. To report results of functional, biochemical and structural s tudies of photoreceptor mitochondria in isolated rat retinas under con ditions of mitochondrial inhibition. Methods. Dark-adapted rat retinas were incubated in a modified Ringer's bicarbonate medium under aerobi c and anaerobic conditions. Several different procedures were used to inhibit mitochondrial function: N-2, 0.01 mM antimycin A, and 1 and 10 mM potassium cyanide (KCN). Measurements were made of lactic acid pro duction, retinal adenosine triphosphate (ATP) content, and receptor po tentials. Morphology of the inner segment mitochondria was examined by electron microscopy. Results. In the presence of N-2, 0.01 mM antimyc in, or 1 mM KCN, lactic acid production was linear throughout the 60- minute period; and the rate was similar for each condition. Retinal AT P content and the amplitude of the receptor potential were also mainta ined at high levels after short-term incubations with either N-2, anti mycin A, or 1 mM KCN. In contrast, use of 10 mM KCN produced an entire ly different set of results. These effects were studied both at the al kaline pH (8.9) found when this concentration of KCN was simply added to bicarbonate-buffered media and at the normal pH (after readjustment ) of 7.4. With 10 mM KCN (pH 8.9), retinal lactate production was seve rely depressed, retinal ATP content was nearly depleted within 5 to 10 minutes, and the amplitude of the receptor potential rapidly declined to a low level. The deleterious effects of 10 mM KCN on these paramet ers were lessened to varying degrees when pH was readjusted to 7.4. El ectron microscopic observations of rat rod inner segments indicated ge nerally excellent survival of these organelles after incubation with e ither N-2, antimycin A, or 1 mM KCN in comparison with their appearanc e under oxygenated conditions. However, the inner segments were signif icantly disrupted after incubation of retinas with 10 mM KCN. Conclusi ons. Findings suggest that the loss of the receptor potential and depl etion of ATP observed within minutes after exposing isolated rat retin as to media containing 10 mM KCN results from the inhibition of both r espiration and glycolysis by this high concentration of KCN. In contra st, when conditions are chosen so that only respiration is impaired (a s with N-2, antimycin A, or 1 mM KCN) photoreceptor cells are resistan t to short-term episodes of mitochondrial inhibition, principally beca use the upregulation of glycolysis generates sufficient ATP to compens ate reasonably well for the loss in mitochondrially produced ATP.