This study explores the importance of creatine kinase (CK) in the regulation of muscle mitochondrial respiration in human subjects depending on their level of physical activity. Volunteers were classified as sedentary, active or athletic according to the total activity index as determined by the Baecke questionnaire in combination with maximal oxygen uptake values (peak ◊ O 2 , expressed in ml min _1 kg _1 ). All volunteers underwent a cyclo-ergometric incremental exercise test to estimate their peak ◊ O 2 and ◊ O 2 at the ventilatory threshold (VT). Muscle biopsy samples were taken from the vastus lateralis and mitochondrial respiration was evaluated in an oxygraph cell on saponin permeabilised muscle fibres in the absence (◊ 0 ) or in the presence (◊ max ) of saturating [ADP]. While ◊ 0 was similar, ◊ max differed among groups (sedentary, 3.7 ± 0.3, active, 5.9 ± 0.9 and athletic, 7.9 ± 0.5 mmol O 2 min _1 (g dry weight) _1). ◊ max was correlated with peak ◊ O 2 (P < 0.01, r = 0.63) and with ◊ T (P < 0.01, r = 0.57). There was a significantly greater degree of coupling between oxidation and phosphorylation (◊ max /◊ 0 ) in the athletic individuals. The mitochondrial K m for ADP was significantly higher in athletic subjects (P < 0.01). Mitochondrial CK (mi-CK) activation by addition of creatine induced a marked decrease in K m in athletic individuals only, indicative of an efficient coupling of mi-CK to ADP rephosphorylation in the athletic subjects only. It is suggested that increasing aerobic performance requires an enhancement of both muscle oxidative capacity and mechanisms of respiratory control, attesting to the importance of temporal co-ordination of energy fluxes by CK for higher efficacy.

Abstract-We have tested the hypothesis that decreased functioning of creatine kinase (CK) at sites of energy production and utilization may contribute to alterations in energy fluxes and calcium homeostasis in congestive heart failure (CHF). Heart failure was induced by aortic banding in 3-week-old rats. Myofilaments, sarcoplasmic reticulum (SR), mitochondrial functions, and CK compartmentation were studied in situ using selective membrane permeabilization of left ventricular fibers with detergents (saponin for mitochondria and SR and Triton X-100 for myofibrils). Seven months after surgery, animals were in CHF. A decrease in total CK activity could be accounted for by a 4-fold decrease in activity and content (Western blots) of mitochondrial CK and a 30% decrease in M isoform of CK (MM-CK) activity. In myofibrils, maximal force, crossbridge kinetics, and ␣-myosin heavy-chain expression decreased, whereas calcium sensitivity of tension development remained unaltered. Myofibrillar CK efficacy was unchanged. Calcium uptake capacities of SR were estimated from the surface of caffeine-induced tension transient (S Ca ) after loading with different substrates. In CHF, S Ca decreased by 23%, and phosphocreatine was 2 times less efficient in enhancing calcium uptake.Oxidative capacities of the failing myocardium measured as oxygen consumption per gram of fiber dry weight decreased by 28%. Moreover, the control of respiration by creatine, ADP, and AMP was severely impaired. Our observations provide evidence that alterations in CK compartmentation may contribute to alterations of energy fluxes and calcium homeostasis in CHF. (Circ Res. 1999;85:68-76.)Key Words: mitochondrial respiration Ⅲ myofibril Ⅲ compartmentation Ⅲ sarcoplasmic reticulum Ⅲ skinned fiber T he mechanisms underlying the decline in cardiac pump function in heart failure are incompletely understood. They lead to a gradual increase in left ventricular (LV) end diastolic pressure and a decrease in systolic pressure. In the past decade attention has been focused on the alterations of the various steps in excitation-contraction coupling and intracellular calcium homeostasis, 1-3 whereas the possible involvement of a mismatch in energy supply and demand has received less attention. 4 -6 It has been shown recently that in human heart failure, there is a generalized alteration of the creatine kinase (CK) system with a decrease in total enzyme activity and velocity and alteration in the isoenzyme pattern, which could contribute to the pathogenesis of heart failure. 5 Moreover, in patients with dilated cardiomyopathy, the phosphocreatine (PCr)/ATP ratio, governed by CK activity, may be a predictor of both total and cardiovascular mortality. 7 However, the precise cellular mechanisms by which altered CK may compromise energy fluxes and contractility are not well understood.CK is an important enzyme involved in energy maintenance and energy transfer in muscle and brain cells. It catalyzes the reversible transfer of a phosphate moiety between ATP and creatine. Four d...