SUBSTRATE-DEPENDENT EFFECTS OF CALCIUM ON RAT RETINAL MITOCHONDRIAL RESPIRATION - PHYSIOLOGICAL AND TOXICOLOGICAL STUDIES

Cytosolic Ca2+ overload may play a key role in the process of lead-ind uced retinal injury and degeneration. We report that retinal calcium c ontent was elevated following developmental and in vitro lead exposure . To determine the concentration-dependent effects of Ca2+ (5-1000 nM) on retinal mitochondrial bioenergetics an isolation procedure was dev eloped. Isolated mitochondria were efficiently coupled; had good respi ratory control ratios with the NAD-linked substrates, glutamate or pyr uvate plus malate (G/M or P/M), and the FAD-linked substrate, succinat e plus rotenone (S/R); and possessed a Na+/Ca2+ exchanger. The major f inding was that at equimolar [Ca2+] greater than or equal to 35 nM, mi tochondria were more sensitive to and exhibited a greater degree of in hibition of coupled and uncoupled respiration with NAD-linked substrat es compared to S/R. At all [Ca2+], decreases in State 3 and uncoupled respiration were similar, thereby eliminating the ATP synthase and ADP /ATP translocase as sites of inhibition and suggesting that opening th e mitochondrial permeability transition pore (MTP) did not contribute to the inhibition. The effects of toxicological [Ca2+] were: (1) block ed by ruthenium red, (2) blocked by dibucaine only in the presence of NAD-linked substrates, and (3) partially reversed by NAD(+) with G/M a fter opening the MTP. Results with G/M suggest that Ca2+ acts on the i nner membrane phospholipase A, to decrease NADH CoQ reductase activity and/or produce a NAD(+) leak, whereas with S/R, Ca2+ may inhibit succ inate dehydrogenase. In conclusion, Ca2+ inhibits retinal mitochondria l ATP production, which may contribute to the retinal cell injury and death observed in developmentally lead-exposed rats. (C) 1994 Academic Press, Inc.