MAXIMAL RATE OF PHOSPHOCREATINE DEPLETION IS DEPENDENT ON THE METABOLIC STATE.
Kent Sahlin · Medicine & Science in Sports & Exercise · 2003
PURPOSE Maximal rate of ATP generation (energetic power) by the creatine kinase (CK) reaction exceeds that of glycolysis and oxidative phosphorylation. However, the effect of phosphocreatine (PCr) depletion on the energetic power has not been elucidated. It is generally assumed that CK reaction is at equilibrium both at rest and during exercise. This is the foundation for calculating the metabolic active form of ADP (ADPfree) from the CK equilibrium. However, the flux of a near equilibrium reaction is dependent on the displacement from equilibrium and for the CK reaction this is mainly achieved by changes in [ADPfree]. The purpose of the present work was to calculate the dependence of CK energetic power on PCr/Cr and [ADPfree]. METHODS Based on a previous model of the kinetics of the CK reaction [Kushmerick, Comp. Biochem. Physiol. 120: 109–123, 1998) the net rate of PCr depletion was calculated as a function of PCr/Cr ratio, pH and [ADPfree]. RESULTS The maximal rate (Vmax) of the reverse CK reaction (ATP + Cr → PCr + ADP + H+) is dependent on pH and varies between 54 (pH = 7.0) and 13% (pH = 6.4) of that of the forward reaction (PCr + ADP + H+ → Cr + ATP). At the conditions prevailing at rest (PCr/Cr = 2.28; pH =7.0) the CK reaction is at equilibrium (i.e. no net flux) when [ADPfree] is 0.02 mM. An increase in ADPfree will increase the rate of the forward reaction and decrease the rate of the reverse reaction and at 0.1 mM the net flux of PCr depletion is almost 40% of the Vmax forward. At the conditions prevailing at fatigue (PCr/Cr = 0.17; pH = 6.4) the same concentration of ADPfree (0.1 mM) will only elicit 4% of Vmax forward. To maintain the same rate of PCr depletion at fatigue ADPfree must increase to 0.55 mM, which probably will impair the contraction process. Discussion: The performed modelling reveals that the maximal energetic power of the CK reaction decreases when PCr/Cr is reduced. Decreases in PCr may therefore limit ATP generation and power output before the whole store of PCr is depleted. This may explain previous findings of reduced exercise power when preceded by submaximal exercise.