THE EFFECTS OF METABOLIC FUEL ON FORCE PRODUCTION AND RESTING INORGANIC-PHOSPHATE LEVELS IN MOUSE SKELETAL-MUSCLE

1. The effect of different metabolic fuels (glucose, pyruvate and lactate) and no exogenous metabolic fuel on force production was studied in isolated mouse soleus and extensor digitorum longus (EDL) muscles. Force was measured, at 25-degrees-C, during isometric tetanic contractions and during contractions with isovelocity stretching and shortening. In parallel experiments, measurements were made of the resting phosphorus metabolite levels using P-31 NMR. 2. In soleus muscles, the isometric tetanic force was potentiated with pyruvate (20 mM) as metabolic fuel, compared with glucose (11 mm), by 17.8 +/- 3.6 % (mean +/- S.E.M., n = 6). The force was the same with no exogenous metabolic fuel, with glucose, or with lactate as metabolic fuel. The force exerted during shortening was also potentiated by pyruvate and by the same proportion as isometric force. However, during rapid stretching there was no force enhancement with pyruvate. The changes in the force seen with pyruvate are qualitatively similar to those produced when inorganic phosphate (P(i)) is lowered in skinned rabbit psoas muscle fibres. 3. We tested whether the P(i) content decreased in the presence of pyruvate by measuring resting P(i) using P-31 NMR spectroscopy. We found that, in soleus muscles, resting P(i) was present with glucose and absent with pyruvate as metabolic fuel, and the effect was reversible. 4. EDL muscles produced the same isometric force whether the metabolic fuel was glucose, pyruvate, lactate or if no exogenous metabolic fuel was supplied. EDL muscles already had P(i) levels below detectability at rest in glucose. There were no changes in the P-31 NMR spectrum with pyruvate as metabolic fuel. 5. It appears therefore that the force potentiation in soleus muscles with pyruvate is due to a lowering of P(i). EDL muscles, which have a very low resting P(i) in glucose, therefore have very little potential for force enhancement by this mechanism.