Fatigue and Recovery in Isolated Frog Sartorius Muscles: The Effects of Bicarbonate Concentration and Associated Potassium Loss

The time course of recovery in isolated frog sartorius muscles was followed by measuring isometric twitch tension after a standard stimulation procedure (400 impulses at 2/s). After stimulation, isometric twitch tension falls to 10–30% of its normal value. Recovery follows three phases: (1) a rapid tension increase within the first 5 min after the end of stimulation which is probably related to the loss of accumulated extracellular potassium; (2) a plateau phase lasting 10–20 min in which a further depression of tension often occurs; (3) a phase of slow tension increase for 30–60 min. In the presence of Ringers solution containing a high bicarbonate concentration (25 mequiv/l), tension recovers to 80–90% of 'rested state' tension in 60 min. In low bicarbonate concentrations (1 mequiv/l) recovery is only 30–40%. Low bicarbonate is associated with a high net potassium efflux during recovery. At a given bicarbonate level small muscles (<100 mg) recover better and lose less potassium per unit weight than larger muscles.