Patterns in mammalian muscle energetics.

A description of cellular energetics of muscular contraction is given in terms of the rates and extents of high-energy phosphate splitting during contractile activity, in terms of high-energy phosphate resynthesis by respiration and net anaerobic glycolysis, and in terms of the associated uptake and/or release of H+. These chemical changes have been studied quantitatively by rapid freeze-clamping methods and by 31P-NMR methods. The pattern of chemical changes in a fast-twitch glycolytic muscle is rapid depletion of phosphocreatine and later ATP levels, cellular acidification, and a much slower rate of resynthesis of high-energy phosphate compounds during the recovery period afterwards than occurs in the slow-twitch oxidative muscles. In steady-state contractile activity below the maximal, graded levels of high-energy phosphates and of cellular respiration are achieved in both fast-twitch and slow-twitch muscles. Within the metabolic range up to the maximal aerobic capacity, which differs several-fold for different fibre types, this gradation is mediated by the creatine kinase reaction and phosphocreatine stores. Thus while the amount of enzyme present and the content of phosphocreatine differs among muscles of different types, the same general energetic function is seen to occur in all muscle cells. The creatine kinase reaction is both an energy reservoir and a buffer preventing large swings in the ATP/ADP ratios.