Disruption of myoglobin in mice induces multiple compensatory mechanisms

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Abstract
Myoglobin may serve a variety of functions in muscular oxygen supply, such as O2 storage, facilitated O2 diffusion, and myoglobin-mediated oxidative phosphorylation. We studied the functional consequences of a myoglobin deficiency on cardiac function by producing myoglobin-knockout (myo−/−) mice. To genetically inactivate the myoglobin gene, exon 2 encoding the heme binding site was deleted in embryonic stem cells via homologous recombination. Myo−/− mice are viable, fertile, and without any obvious signs of functional limitations. Hemoglobin concentrations were significantly elevated in myo−/− mice. Cardiac function and energetics were analyzed in isolated perfused hearts under resting conditions and during β-adrenergic stimulation with dobutamine. Myo−/− hearts showed no alteration in contractile parameters either under basal conditions or after maximal β-adrenergic stimulation (200 nM dobutamine). Tissue levels of ATP, phosphocreatine (31P-NMR), and myocardial O2 consumption were not altered. However, coronary flow {6.4 ± 1.3 ml⋅min−1⋅g−1 [wild-type (WT)] vs. 8.5 ± 2.4 ml⋅min−1⋅g−1 [myo−/−]} and coronary reserve [17.1 ± 2.1 (WT) vs. 20.8 ± 1.1 (myo−/−) ml⋅min−1⋅g−1 were significantly elevated in myo−/− hearts. Histological examination revealed that capillary density also was increased in myo−/− hearts [3,111 ± 400 mm−2 (WT) vs. 4,140 ± 140 mm−2 (Myo−/−)]. These data demonstrate that disruption of myoglobin results in the activation of multiple compensatory mechanisms that steepen the pO2 gradient and reduce the diffusion path length for O2 between capillary and the mitochondria; this suggests that myoglobin normally is important for the delivery of oxygen.