Key Points
- To evaluate the direct effects of physiological MgADP concentrations on maximum isometric force and unloaded shortening velocity in single skinned fast- and slow-twitch muscle fibers.
- Tested single skinned fiber segments from rabbit fast-twitch (psoas) and slow-twitch (soleus) muscles across controlled MgADP concentrations (up to 200 µM) buffered by creatine kinase, phosphocreatine, and creatine at pH 7.1 and 12 °C.
- Calculated the apparent equilibrium constant of creatine kinase (K' = 260 ± 3) to precisely regulate ADP without confounding changes in pH or inorganic phosphate.
- Assessed mechanical function via Ca2+-activated isometric force and unloaded shortening velocity (Vus) in the presence of 290 U/ml exogenous creatine kinase.
- At the highest physiological ADP concentration (200 µM), normalized force was 96.6 ± 1.7% in psoas fibers (n = 6) and 93.7 ± 2.8% in soleus fibers (n = 6).
- Unloaded shortening velocity (Vus) at 200 µM ADP decreased slightly to 80.4 ± 2.4% in psoas (n = 6) and 91.3 ± 7.7% in soleus (n = 6).
- Creatine alone reduced force and velocity non-specifically, while altered physiological ADP had little to no direct effect on fiber mechanics in well-buffered conditions.
Structured PICO
PPopulationSingle skinned fiber segments from rabbit fast-twitch (psoas, n=6) and slow-twitch (soleus, n=6) muscles
IInterventionPhysiological MgADP levels (up to 200 microM) controlled by varying creatine in solutions
CComparatorControl levels of MgADP
OOutcomeMaximum Ca(2+)-activated isometric force and unloaded shortening velocity (Vus)surrogate
Over the physiological range, altered ADP has little or no effect on force or unloaded shortening velocity in well-buffered conditions, suggesting other factors are responsible for muscular fatigue.