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Due to glycolysis, anaerobic ATP supports normal electrical activity and allows only a reduced but sustained mechanical activity; however, frog myocardium metabolism is mainly aerobic dependent. Krebs cycle provides ATP for both mechanical and electrical acitivites even when glycolysis is inhibited. The close correlation between amplitude of contraction and metabolic state suggests that ATP is a limiting factor of mechanical activity even in normal conditions. Therefore it is necessary to use oxygen and to add pyruvate which partly compensate for the decrease of mechanical activity observed during the occurrence of hypodynamic state. Moreover equilibrating the oncotic pressure prevents hypodynamia and considerably improves the experimental conditions.

The importance of respecting the ionic equilibrium of cardioplegic solutions was studied experimentally in rat papillary muscle by observing the mechanical and metabolic changes after altering the ionic composition of the washing fluid. The first part of the study demonstrated the nature of the "rigor" of sustained tension induced by the inhibition of the respiratory activity of the mitochondria: this observation showed that this tension was essentially related to the energy deficit induced by poisoning. In the second part, it was shown that calcium overload induced by changing the ionic composition of the nutrient fluid (replacing some of the Na+ ions by K+ or Li+), that is to say related to dysfunction of the Na+-Ca++ exchange induced a contraction which also rapidly evolved to a tension of rigor. This effect was accompanied by a significant reduction in the adenosine triphosphate (ATP) and creatine phosphate (CP) content. The mechanical consequences of prolonged hypoxia or calcium overload are directly related to the reduction in energy reserves producing a marked deficit of ATP in the myofibrils, and resulting in a stable but reversible increase in cardiac muscle rigidity (or rigor). The prevention of calcium overload during the use of cardioplegic solutions is discussed in the context of metabolic myocardial protection. The inhibitory substances of calcium influx such as the calcium antagonist drugs or Mg++ may have an important role to play, in addition to the maintenance of ionic concentrations, especially that of sodium, close to those of the external fluid.