Glycolytic activation at the onset of contractions in isolated Xenopus laevis single myofibres

Intracellular pH (pH i ) was measured in isolated Xenopus laevis single myofibres at the onset of contractions, with and without glycolytic blockade, to investigate the time course of glycolytic activation. Single myofibres ( n = 8; CON) were incubated in 2′,7′‐bis(carboxyethyl)‐5(6)‐carboxyfluorescein acetoyxmethyl ester (10 μ m ; for fluorescence measurement of pH i ) and stimulated for 15 s at 0.67 Hz in anoxia in the absence (control condition; CON) and presence of a glycolytic inhibitor (1 m m iodoacetic acid; IAA). Intracellular pH i and tension were continuously recorded, and the differences in pH i between conditions were used to estimate the activation time of glycolysis. An immediate and steady increase in pH i (initial alkalosis) at the onset of contractions was similar between CON and IAA trials for the first 9 s of the contractile bout. However, from six contractions (∼10 s) throughout the remainder of the bout, IAA demonstrated a continued rise in pH i , in contrast to a progressive decrease in pH i in CON ( P < 0.05). These results demonstrate, with high temporal resolution, that glycolysis is activated within six contractions (10 s at 0.67 Hz) in single Xenopus skeletal muscle fibres.