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-induced retinal injury and degeneration. We report that retinal calcium content 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 developed. Isolated mitochondria were efficiently coupled; had good respiratory control ratios with the NAD-linked substrates, glutamate or pyruvate plus malate (G/M or P/M), and the FAD-linked substrate, succinate plus rotenone (S/R); and possessed a Na+/Ca2+ exchanger. The major finding was that at equimolar [Ca2+] > or = 35 nM, mitochondria were more sensitive to and exhibited a greater degree of inhibition of coupled and uncoupled respiration with NAD-linked substrates 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 the mitochondrial permeability transition pore (MTP) did not contribute to the inhibition. The effects of toxicological [Ca2+] were: (1) blocked by ruthenium red, (2) blocked by dibucaine only in the presence of NAD-linked substrates, and (3) partially reversed by NAD+ with G/M after opening the MTP. Results with G/M suggest that Ca2+ acts on the inner membrane phospholipase A2 to decrease NADH CoQ reductase activity and/or produce a NAD+ leak, whereas with S/R, Ca2+ may inhibit succinate dehydrogenase. In conclusion, Ca2+ inhibits retinal mitochondrial ATP production, which may contribute to the retinal cell injury and death observed in developmentally lead-exposed rats.
Medrano
C J
CJ
College of Optometry, University of Houston, Texas 77204-6052.
Fox
D A
DA
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000494:pharmacology* / Q000502:physiology; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000008:administration & dosage; D004734:Energy Metabolism / Q000187:drug effects / Q000502:physiology; D005260:Female; D066298:In Vitro Techniques; D007854:Lead / Q000032:analysis; D008854:Microscopy, Electron; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D010085:Oxidative Phosphorylation / Q000187:drug effects; D010101:Oxygen Consumption / Q000187:drug effects; D051381:Rats; D011919:Rats, Inbred Strains; D012160:Retina / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D013347:Subcellular Fractions / Q000201:enzymology; D013379:Substrate Specificity
D007854:Lead; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D000255:Adenosine Triphosphate; D002118:Calcium