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Department of Biological Chemistry, University of California, Davis, California 95616-8635.
The 1H-NMR signal of the
proximal histidyl-N
H of deoxymyoglobin is detectable in
the in situ rat myocardium and can reflect the intracellular
PO2. Under basal normoxic conditions, the
cellular PO2 is sufficient to saturate
myoglobin (Mb). No proximal histidyl signal of Mb is detectable. On
ligation of the left anterior descending coronary artery, the Mb signal
at 78 parts/million (ppm) appears, along with a peak shoulder assigned
to the corresponding signal of Hb. During dopamine infusion up to 80 µg · kg
1 · min1, both the
heart rate-pressure product (RPP) and myocardial oxygen consumption
(M
O2) increase by about a
factor of 2. Coronary flow increases by 84%, and O2
extraction (arteriovenous O2 difference) rises by 31%.
Despite the increased respiration and work, no deoxymyoglobin signal is
detected, implying that the intracellular O2 level still saturates MbO2, well above the PO2
at 50% saturation of Mb. The phosphocreatine (PCr) level
decreases, however, during dopamine stimulation, and the ratio of the
change in Pi over PCr (
Pi/PCr) increases by
0.19. Infusion of either pyruvate, as the primary substrate, or
dichloroacetate, a pyruvate dehydrogenase activator, abolishes the
change in Pi/PCr. Intracellular O2 supply
does not limit MO2, and the
role of ADP in regulating respiration in rat myocardium in vivo remains
an open question.
myoglobin; nuclear magnetic resonance; respiration; bioenergetics; heart
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