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NMR Laboratory for Physiological Chemistry, Cardiovascular Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 02115
Endogenous
nitric oxide (eNO) modulates tissue respiration. To test whether eNO
modulates myocardial O2 consumption
(M
O2), ATP synthesis, and metabolic
efficiency, we used isolated isovolumic guinea pig hearts perfused at a
constant flow. N
-nitro-L-arginine
(L-NNA; 5 × 10
5 mol/l) was used to
inhibit eNO production. M
O2 was measured at different levels of cardiac work, estimated as the rate-pressure product (RPP). ATP content and synthesis rate were determined using
31P NMR and magnetization transfer during high cardiac
work. L-NNA increased coronary vascular resistance (19 ± 3%, P < 0.05) and M
O2 (12 ± 3%, P < 0.05) without an increase in the RPP. In contrast, vehicle infusion
resulted in insignificant changes in coronary vascular resistance
(3 ± 2%, P > 0.05) and
M
O2 (
2 ± 1%, P > 0.05). Compared with vehicle, L-NNA caused a higher
M
O2 both during KCl arrest
(L-NNA 5.6 ± 0.5 vs. vehicle 3.0 ± 0.4 µmol · min
1 · mg dry wt
1,
P < 0.05) and during increased cardiac work elicited
by elevating perfusate Ca2+, indicating an upward shift in
the relationship between contractile performance (measured as RPP) and
M
O2. However, neither ATP contents nor
ATP synthesis rates were different in the two groups during high
cardiac work. Thus, because inhibition of eNO production by
L-NNA increased M
O2 without
a change in the ATP synthesis rate, these data suggest that eNO
increases myocardial metabolic efficiency by reducing
M
O2 in the heart.
nuclear magnetic resonance; N
-nitro-L-arginine
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