Implication of CO inactivation on myoglobin function

Myoglobin (Mb) has a purported role in facilitating O₂ diffusion in tissue, especially as cellular PO₂ drops or the respiration demand increases. Inhibiting Mb with CO under conditions that accentuate the facilitated diffusion role should then elicit a significant physiological response. In one set of experiments, the perfused myocardium received buffer with decreasing PO₂ (225, 129, and 64 mmHg). Intracellular PO₂ declined, as reflected in the ¹H NMR Val E11 signal of MbO₂ (67%, 32%, and 18%). The addition of 6% CO further reduced the available MbO₂ (11%, 9%, and 7%), as evidenced by the decline of the MbO₂ Val E11 signal intensity at –2.76 ppm. In a second set of experiments, electrical stimulation increased the heart rate (300, 450, and 540 beats/min) and correspondingly the O₂ consumption rate (MO₂). Intracellular PO₂ also declined, as reflected in the slight drop in the MbO₂ signal (100%, 96%, and 82%). MO₂ increased (100%, 114%, 165%). The addition of 3% CO in the stimulated hearts further decreased the available MbO₂ (46%, 44%, and 29%). In all cases, CO inactivation of Mb does not induce any change in the respiration rate, contractile function, and high-energy phosphate levels. Moreover, the MbCO/MbO₂ partition coefficient shifts dramatically from its in vitro value during hypoxia and increased work. The observation suggests a modulation of an intracellular O₂ gradient. Overall, the experimental observations provide no evidence of a facilitated diffusion role for Mb in perfused myocardium and implicate a physiologically responsive intracellular O₂ gradient. nuclear magnetic resonance; respiration; carbon monoxide; myocardium; oxidative phosphorylation