Quantification of myoglobin deoxygenation and intracellular partial pressure of O2 during muscle contraction during haemoglobin-free medium perfusion

Although the O-2 gradient regulates O-2 flux from the capillary into the myocyte to meet the energy demands of contracting muscle, intracellular O-2 dynamics during muscle contraction remain unclear. Our hindlimb perfusion model allows the determination of intracellular myoglobin (Mb) saturation (S-MbO2) and intracellular oxygen tension of myoglobin (P-MbO2) in contracting muscle using near infrared spectroscopy (NIRS). The hindlimb of male Wistar rats was perfused from the abdominal aorta with a well-oxygenated haemoglobin-free Krebs-Henseleit buffer. The deoxygenated Mb (Delta [deoxy-Mb]) signal was monitored by NIRS. Based on the value of Delta[deoxy-Mb], S-MbO2 and P-MbO2 were calculated, and the time course was evaluated by an exponential function model. Both S-MbO2 and P-MbO2 started to decrease immediately after the onset of contraction. The steady-state values of S-MbO2 and P-MbO2 progressively decreased with relative work intensity or muscle oxygen consumption. At the maximal twitch rate, S-MbO2 and P-MbO2 were 49% and 2.4 mmHg, respectively. Moreover, the rate of release of O-2 from Mb at the onset of contraction increased with muscle oxygen consumption. These results suggest that at the onset of muscle contraction, Mb supplies O-2 during the steep decline in P-MbO2, which expands the O-2 gradient to increase the O-2 flux to meet the increased energy demands.