Intracellular pH and relaxation of frog muscle

The experiments were done to investigate the effects of intracellular pH (pHi) on relaxation of intact skeletal muscle of the frog. The pHi was changed in a controlled way, independent of other metabolite levels, by changing the extracellular concentration of the permeant acid carbon dioxide. In parallel experiments pH-sensitive microelectrodes were used to measure the change in pHi. The results show that relaxation was slower when pHi was more acid. The size of this effect depended strongly on tetanus duration. After a brief tetanus (0.5 sec at 0 degrees C) the effect was small, but after a tetanus of 5 sec or more, relaxation was greatly prolonged. Experiments with paired tetani (conditioning and test) suggested that changes in the levels of other metabolites (inorganic phosphate, etc.) were not responsible for this effect. Relaxation from a test tetanus was slower if a conditioning tetanus was given, but was independent of muscle length, and thus independent of filament overlap and the amount of ATP cleavage, during conditioning. Mechanisms that could explain these effects of pHi and tetanus duration include: (1) slow Ca2+ uptake by the sarcoplasmic reticulum at acid pH and pH-insensitive Ca2+ binding by parvalbumin, (2) removal of Ca2+ from the myoplasm occurring more uniformly along the length of the fibres when pHi is acid.