The effect of bicarbonate concentration on fatigue and recovery in isolated rat diaphragm muscle.

Diaphragm strips from young rats (45--60 g) about 0.3 mm thick were fatigued by tetanic stimulation at a train repetition rate of 2 HZ for 3 min. The isometric tension developed was measured during fatigue and recovery in solutions containing 25, 10, or 2 mM bicarbonate at both 37 and 30 degrees C. Tension fell during fatigue to between 20 and 30% of the initial value and this was not significantly influenced by external bicarbonate concentration or temperature over the range considered. Recovery of tension was complete and rapid (t1/2 < 1 min) in 25 mM bicarbonate at both temperatures. In 10 and 2 mM bicarbonate recovery was slowed (t1/2 3.5 and 7 min, respectively, at 30 degrees C, 1.6 and 4.5 min at 37 degrees C) and incomplete (85 and 72% at 30 degrees C, 82 and 61% at 37 degrees C). Muscle creatine phosphate fell during fatigue but was completely restored within 4 min at 30 degrees C in either 2 or 25 mM HCO3. Lactate increased less in muscles fatigued in 2 mM HCO3 and fell at a slower rate during recovery. The results seem to exclude intracellular creatine phosphate concentration as a major determinant in recovery. The evidence suggests that external bicarbonate can affect the recovery of tension following fatigue by altering intracellular acid-base balance.