Influence of physiological L(+)-lactate concentrations on contractility of skinned striated muscle fibers of rabbit

These experiments investigated the effects of physiological concentrations of L(+)-lactate on the contractility of chemically shinned rabbit fast-twitch psoas, slow-twitch soleus, and cardiac muscles at pH 7. L(+)-Lactate depressed maximal calcium-activated force (F-max) of all muscles studied within the range of 5-20 (slow-twitch muscle) or 5-25 mM (fast-twitch and cardiac muscles), F-max of fast-twitch fibers was inhibited to the greatest degree (9% in K-2 creatine phosphate solutions). In all of these muscle types, F-max returned to control levels as L(+)-lactate was increased to 30-50 mM. Substitution of neither D-lactate nor propionate for L(+)-lactate significantly altered F-max. In addition, with the exception of fast-twitch muscle (where the Hill coefficient, decreased), L(+)-lactate concentrations, which maximally inhibited F-max, did not affect the force vs. pCa relationship of muscles tested. These results demonstrate that L(+)-lactate significantly contributes to the depression of muscle function noted during lactic acidosis, directly inhibiting F-max of the contractile apparatus. This contribution is maximal in fast-twitch muscle where L(+)-lactate is responsible for as much as one-third of the depressant effect on F-max of the contractile apparatus noted during lactic acidosis.