Anaerobic ATP provision, glycogenolysis and glycolysis in rat slow-twitch muscle during tetanic contractions.

Rat soleus muscles were tetanically stimulated in situ with an occluded circulation to examine anaerobic adenosine triphosphate (ATP) provision and the regulation of glycolytic ATP production. Soleus muscles were stimulated for 30-60 s at 1 Hz with 100-200 ms trains (40-80 Hz). Muscles were sampled pre- and post-stimulation for measurements of pH, high energy phosphates and glycolytic intermediates. Total ATP provision by the slow oxidative fibres was 65-121 mumol/g dry muscle and 27-35% of the amount produced by fast glycolytic fibres. Contributions to total ATP provision in the initial 30 contractions were: phosphocreatine, 71%; glycolysis, 28%; and endogenous ATP, 1%. Following 60 contractions the contributions were 45-54%, 44-51% and 2-4%, respectively. During the initial 30 contractions, glycogenolysis (phosphorylase activity) and glycolysis [phosphofructokinase (PFK) activity] were similar as glucose-6-phosphate (G-6-P) and fructose-6-phosphate (F-6-P) did not accumulate. Small accumulations of PFK deinhibitors inorganic phosphate, adenosine diphosphate, adenosine monophosphate and fructose-1,6-diphosphate appeared to account for the PFK activity. In the final 30 contractions, phosphorylase activity increased above PFK as G-6-P and F-6-P accumulated. PFK activity and glycolytic ATP production also increased despite increasing hydrogen ion concentration [H+]. During intense tetanic stimulation of soleus muscle, glycolytic ATP production is initially limited by a low glycogen phosphorylase activity. The activity of PFK increases during in situ contraction through the accumulation of deinhibitors, despite increasing [H+].