Fig. 4. Recordings of the respiration rate in permeabilized myocardial fibers activated by endogenous ADP production in MgATPase reactions. Traces show the rate of change of oxygen concentration in time in an oxygraph cell. Respiration rates were measured in the presence of 5mmoll^-1 glutamate plus 2mmoll^-1 malate, as described in Materials and methods. Addition of 2mmoll^-1 ATP, and various final concentrations of pyruvate kinase (PK) in the presence of 2mmoll^-1 phosphoenolpyruvate (PEP) in the medium are indicated. At the end of experiments, 20mmoll^-1 creatine was added. Arrows show the time of addition. The results show some inhibitory effect of the competitive pyruvate kinase (PK)–PEP system for endogenous ADP on the respiration rate and a stimulatory effect of creatine, due to coupled creatine kinase reaction, in the presence of PK. Only minor inhibition of respiration by very high PK activity (20i.u.ml^-1) demonstrates compartmentation and direct channeling of endogenous ADP. These effects of direct channeling are increased after activation of mitochondrial creatine kinase.

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