Contractile Performance And Energy Utilization Of Skeletal Muscle: Creatine Kinase And Acto-myosin Atpase

Creatine kinase (CK) is bound in the sarcomere near sites of ATP consumption via acto-myosin ATPase and research in cardiac muscle has found that PCr can alter contractile performance (maximal isometric force and Ca2+ sensitivity). Based on this evidence, CK and acto-myosin ATPase may be coupled in skeletal muscle. PURPOSE: To compare the effects of exogenous ATP and endogenously generated ATP on contractile performance and energy utilization in skeletal muscle. METHODS: Extensor digitorum longus skeletal muscle fibers of the rat were chemically skinned and exposed to solutions containing either high (2 mM) or low (0.1 mM) ATP with and without substrates of the CK reaction, PCr (10 mM) and ADP (2 mM). Fibers were then exposed to a physiological range of Ca2+ while maintaining initial concentrations of ATP, ADP, and/or PCr, depending on condition. An indirect assessment the pathway of ADP was also determined by using a coupled enzyme system of pyruvate kinase and lactate dehydrogenase. This pathway was assessed by monitoring the rate of change of NADH fluorescence and reported in μM/sec. RESULTS: When skinned fibers were provided a limited supply of [ATP], this resulted in increased Ca2+ sensitivity compared to the high [ATP] condition (pCa50 7.2 ± 0.1 vs. pCa 6.3 ± 0.1, P < 0.05). Calcium sensitivity was restored with the addition of PCr to low ATP solutions (pCa50 6.2 ± 0.1). This was also possible with only CK reaction substrates (ADP, PCr) in the absence of ATP. Addition of PCr also lowered the rate of change of NADH fluorescence which indirectly identified that ADP produced by acto-myosin ATPase was recycled by the CK reaction (194 ± 77 vs. 52 ± 10 μM/sec, P < 0.05). Maximal force measurements were not significantly different between conditions with the exception of the ADP and PCr only condition showing significantly higher force values than the low [ATP] condition (3.96 ± 0.6 vs 0.8 ± 0.69 kN/m2). Based on these findings, endogenous CK activity in skinned skeletal muscle fibers is sufficient to permit normal function of the contractile apparatus. CONCLUSION: Changing energetic demands on skeletal muscle (increased Ca2+) and substrate availability altered force production and Ca2+ sensitivity. This suggests an important role for endogenous CK on ATP regeneration in skeletal muscle.