Myoglobin facilitated oxygen diffusion maintains mechanical function of mammalian cardiac muscle

Summary: Myoglobin, an intracellular iron containing protein that binds oxygen reversibly, has been shown in model systems to facilitate the diffusion of oxygen and thereby maintain the mechanical function of exercising canine skeletal muscle and of hypoxic benthic fish hearts. Since no such role has yet been established for mammalian cardiac muscle small diameter (≤0.70 mm) isolated kitten papillary muscles were stimulated at 24·min^-1 under isometric conditions in a physiological bath maintained at 30°C with an oxygen tension of ≈450 mm Hg (59.8 kPa) to obtain a level of oxygenation just adequate to meet the metabolic needs of the muscles, as confirmed experimentally. Myoglobin was inactivated by adding 2 × 10^-3 mol·litre^-1 sodium nitrite to the bath to abolish the facilitated diffusion of oxygen in the presence or absence of glycolytic blockade by 10^-4 mol·litre^-1sodium iodoacetate. This resulted in a 22(8)% (with blockade) or 10(3)% (without blockade) decrease (p<0.05) in the maximal rate of relaxation (-dT/dt_max) of the papillary muscles. Since die depression in mechanical function was reversible by increasing the bath oxygen tension to ≈600 mm Hg (79.8 kPa) it is concluded that the myoglobin facilitated diffusion of oxygen plays a role in maintaining the mechanical function of mammalian cardiac muscle under normal conditions. Furthermore, the maximal rate of relaxation of cardiac muscle is a sensitive indicator of the presence of hypoxia.