Relations between muscle pH and important biochemical parameters during the postmortem changes in mammalian muscles

The relations between muscle pH and the levels of lactate, PC, ATP and free ADP during the development of rigor mortis have been studied in normal beef, sheep, pig and rabbit muscles and, in some cases, in the muscles from stimulated carcasses. There are exponential relations between the PC levels and both the lactate levels and the pH, but these relations have very different slope constants in different muscles, depending on whether the particular muscle was quiescent or active during slaughtering. The slope constants can vary by a factor of three, being very large in quiescent muscles and small in active muscles or in muscles from stimulated carcasses. There is a linear relation between the relative pH or the relative lactate level, on the one hand, and the relative ATP level on the other, which applies generally to all the muscles studied, independent of the extent of glycolysis. In quiescent muscles, the free ADP level increases to about one-tenth of the initial ATP level during the later stages of glycolysis, but only to about one-hundredth of this level in active or stimulated muscles. In quiescent muscles the ATP level begins to decline only at very low levels of PC whereas, in active muscles, the PC level at which this decline begins is some three times greater. Some of the aspects of the differing relations between parameters in different muscles may be explained in terms of differing initial amounts of phosphorylase a.