A P-31-NMR STUDY OF TISSUE RESPIRATION IN WORKING DOG MUSCLE DURING REDUCED O-2 DELIVERY CONDITIONS

To investigate the role of tissue oxygenation as one of the control factors regulating tissue respiration, P-31-nuclear magnetic resonance spectroscopy (P-31-NMR) was used to estimate muscle metabolites in isolated working muscle during varied tissue oxygenation conditions. O2 delivery (muscle blood flow x arterial O2 content) was varied to isolated in situ working dog gastrocnemius (n = 6) by decreases in arterial Po, (hypoxemia; H) and by decreases in muscle blood flow (ischemia; 1). O2 uptake (Vo2) was measured at rest and during work at two or three stimulation intensities (isometric twitch contractions at 3, 5, and occasionally 7 Hz) during three separate conditions: normal O2 delivery (C) and reduced O2 delivery during H and I, with blood flow controlled by pump perfusion. Biochemical metabolites were measured during the last 2 min of each 3-min work period by use of P-31-NMR, and arterial and venous blood samples were drawn and muscle blood flow measured during the last 30 s of each work period. Muscle [ATP] did not fall below resting values at any work intensity, even during O2-limited highly fatiguing work, and was never different among the three conditions. Muscle O2 delivery and Vo2 were significantly less (P < 0.05) at the highest work intensities for both I and H than for C but were not different between H and I. As Vo2 increased with stimulation intensity, a larger change in any of the proposed regulators of tissue respiration (ADP, P(i), ATP/ADP - P(i), and phosphocreatine) was required during H and I than during C to elicit a given Vo2, but requirements were similar for H and 1. These results suggest that 1) the sensitivity of mitochondrial respiration to the proposed regulators of tissue respiration can be altered by the level of tissue oxygenation and 2) under the conditions of this study, the bioenergetics of H and I were similar.