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University of Wisconsin Childrens Diabetes Center, Madison, Wisconsin 53706
Succinic acid methyl esters
are potent insulin secretagogues in rat pancreatic islets, but they do
not stimulate insulin release in mouse islets. Unlike rat and human
islets, mouse islets lack malic enzyme and, therefore, are unable to
form pyruvate from succinate-derived malate for net synthesis of
acetyl-CoA. Dimethyl-[2,3-14C]succinate is metabolized in
the citric acid cycle in mouse islets to the same extent as in rat
islets, indicating that endogenous acetyl-CoA condenses with
oxaloacetate derived from succinate. However, without malic enzyme, the
net synthesis from succinate of the citric acid cycle intermediates
citrate, isocitrate, and
-ketoglutarate cannot occur. Glucose and
other nutrients that augment
-ketoglutarate formation are
secretagogues in mouse islets with potencies similar to those in rat
islets. All cycle intermediates can be net-synthesized from
-ketoglutarate. Rotenone, an inhibitor of site I of the electron
transport chain, inhibits methyl succinate-induced insulin release in
rat islets even though succinate oxidation forms ATP at sites II and
III of the respiratory chain. Thus generating ATP, NADH, and
anaplerosis of succinyl-CoA plus the four-carbon dicarboxylic acids of
the cycle and its metabolism in the citric acid cycle is insufficient
for a fuel to be insulinotropic; it must additionally promote
anaplerosis of
-ketoglutarate or two intermediates interconvertible
with
-ketoglutarate, citrate, and isocitrate.
methyl esters of succinate; rotenone; insulin release; citric acid cycle; methyl pyruvate
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