The role of oxygen in determining phosphocreatine onset knietics in exercising humans

Abstract

31P-magnetic resonance spectroscopy was used to study phosphocreatine (PCr) onset kinetics in exercising human gastrocnemius muscle under varied fractions of inspired O2 (FIO2). Five male subjects performed three identical work bouts (5 min duration; order randomised) at a submaximal workload while breathing 0.1, 0.21 or 1.0 FIO2. Either a single or double exponential model was fitted to the PCr kinetics. The phase I Ä (0.1, 38.6 ± 7.5; 0.21, 34.5 ± 7.9; 1.0, 38.6 ± 9.2 s) and amplitude, A1 (0.1, 0.34 ± 0.03; 0.21, 0.28 ± 0.05; 1.0, 0.28 ± 0.03,% fall in PCr) were invariant (both P> 0.05) across FIO2 trials. The initial rate of change in PCr hydrolysis at exercise onset, calculated as A1/Ä1 (%PCr reduction s1), was the same across FIO2 trials. A PCr slow component (phase II) was present at an FIO2 of 0.1 and 0.21; however, breathing 1.0 FIO2 ablated the slow component. The onset of the slow component resulted in a greater (Pd 0.05) overall percentage fall in PCr (both phase I and II) as FIO2 decreased (0.43 ± 0.05, 0.34 ± 0.05, 0.28 ± 0.03) for 0.1, 0.21 and 1.0 FIO2, respectively. These data demonstrate that altering FIO2 does not affect the initial phase I PCr onset kinetics, which supports the notion that O2 driving pressure does not limit PCr kinetics at the onset of submaximal exercise. Thus, these data imply that the manner in which microvascular and intracellular PO2 regulates PCr hydrolysis in exercising muscle is not due to the initial kinetic fall in PCr at exercise onset.

Across the rest to work transition, [ATP] in working muscle is maintained by phosphocreatine (PCr) hydrolysis in a reaction catalysed by creatine kinase (CK). After a few seconds (if not immediately), both glycolysis (Connett et al. 1990; Howlett et al. 1999) and oxidative phosphorylation (Bangsbo, 2000; Behnke et al. 2003; Kindig et al. 2003) are activated. A direct proportionality between the kinetics of pulmonary O2 and PCr hydrolysis has been demonstrated previously (Mahler, 1985; Meyer, 1988), and thus the time course of pulmonary O2 onset kinetics has been related to changing [PCr] in muscle at the start of exercise (Marsh et al. 1993; Barstow et al. 1994; McCann et al. 1995; McCreary et al. 1996; Rossiter et al. 1999). In addition, the steady state level to which PCr falls is correlated positively to the steady state level of O2 during exercise (Mahler, 1985; Meyer, 1988). These data suggest that PCr hydrolysis (specifically the increase in inorganic phosphate) is a potent regulator of oxidative phosphorylation (Meyer, 1988), and that PCr hydrolysis and regulation of oxidative phosphorylation are tightly coupled (Meyer & Foley, 1996).

Our laboratory has demonstrated previously that the tight coupling between the steady state levels of PCr depletion and O2 can be dissociated by altering O2 availability (specifically, microvascular and intracellular PO2; Richardson et al. 1999) during exercise by breathing varied fractions of inspired oxygen (FIO2) (Haseler et al. 1998). Additionally, pulmonary O2 kinetics have been modulated in an O2-dependent manner by altering the FIO2 (Hughes et al. 1968; Linnarsson et al. 1974; Engelen et al. 1996) although other investigations have reported invariant O2 kinetics in the face of altered FIO2 (Linnarsson, 1974; Hughson & Kowalchuk, 1995). To date, whether PCr onset kinetics are O2 dependent has not been tested. Given the tight relationship between pulmonary O2 and PCr hydrolysis, one possible scenario is that PCr kinetics will follow suit with O2 kinetics (Hughes et al. 1968; Linnarsson et al. 1974; Engelen et al. 1996) and be slowed as FIO2 is reduced. Alternatively, despite the fact that O2 kinetics can be slowed with hypoxia due to the necessity for O2 to serve as the terminal electron acceptor within the electron transport chain, PCr hydrolysis is catalysed by CK in a process largely dependent upon the metabolite signals arising from the contractile sites. Thus, regardless of O2 availab