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Division of Cardiothoracic Surgery, University of Wisconsin School of Medicine, Madison, Wisconsin 53792-0001; and Department of Physiology, Uniformed Services University of the Health Sciences, Bethesda, Maryland 20814-4799
Pyruvate has been shown to be a metabolic inotrope
in the myocardium. In millimolar concentrations, it has been shown to
increase both myocardial phosphorylation potential and the cytosolic
[NAD+]-to-[NADH]
ratio. To determine if changes in these parameters can alter
intracellular Ca2+ concentration
([Ca2+]i)
and hence contractile function,
Ca2+ transients and cell
shortening (CS) were measured in isolated rat ventricular myocytes
superfused with a physiological
N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer (11 mmol/l glucose) with and without additional pyruvate, L-lactate, acetate, or isoproterenol. The addition of 5 mmol/l pyruvate resulted in a 33% increase in CS and a 39% increase
in systolic
[Ca2+]i.
These pyruvate effects were 70% of those observed with 100 nmol/l
isoproterenol. The mitochondrial monocarboxylate transport inhibitor
-cyano-4-hydroxycinnamate (250 µmol/l) strongly inhibited pyruvate
inotropy, suggesting a substantial obligatory coupling between pyruvate
inotropism and its oxidation by the mitochondria. A possible role of
the cytosolic
[NAD+]-to-[NADH]
ratio was assessed by comparing the effects of 20 mmol/l
L-lactate to those of equimolar pyruvate. In contrast to 20 mmol/l pyruvate, excess L-lactate failed to appreciably
increase CS or systolic
[Ca2+]i.
The findings imply that, at levels substantially above 5 mmol/l, a
portion of pyruvate inotropism might be due to extreme cytosolic [NAD+]-to-[NADH]
ratios. This study is the first evidence that augmented [Ca2+]i
transients are most likely the mechanism of cardiac pyruvate inotropism.
mitochondria; sarcoplasmic reticulum
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