The effect of glucose loading on steady-state lactate levels and contraction in the isolated rat diaphragm

Isolated rat diaphragm strips were used to investigate two questions: (a) can the steady-state level of lactate in contracting muscles be increased by glucose loading resulting from the addition of increased external glucose (50 mM) in the presence of insulin (10 mU/mL)? (b) is the isometric tension developed by muscles contracting at different frequencies (0.125 to 1.0 Hz) affected by glucose loading? The results show that lactate levels in contracting muscles are increased by glucose loading over the whole range of contraction frequencies studied. Suppression of isometric contraction increases with contraction frequency but the extent of the suppression is not influenced by glucose loading. Steady-state lactate levels are well correlated with suppression in glucose-loaded muscles (r = 0.89) but not well correlated with suppression in normal muscles (r = 0.46). Isometric tension in the steady-state condition is well correlated with creatine phosphate levels (r = 0.98, 0.92 in glucose-loaded and normal muscles) and reasonably well with ATP (r = 0.88, 0.86, glucose loaded and normal). The increase in resting tension seen during development of steady-state conditions is reduced by glucose loading. It is concluded that several factors may contribute to the suppression of tension in contracting muscle but metabolic product inhibition, at least by products of glycolysis, does not normally play an important part in the isolated rat diaphragm preparation.