Cj. Medrano et Da. Fox, SUBSTRATE-DEPENDENT EFFECTS OF CALCIUM ON RAT RETINAL MITOCHONDRIAL RESPIRATION - PHYSIOLOGICAL AND TOXICOLOGICAL STUDIES, Toxicology and applied pharmacology, 125(2), 1994, pp. 309-321
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.