A NOVEL REGULATORY ROLE FOR THE LACTATE ION ON EXCITABILITY AND FORCE

We examined the effect of lactate (lac) per se on excitability and tetanic force development. Rat soleus muscles were incubated in KR‐buffer (95% O 2 ) containing 20 mM Na‐methanosulfonate (MS). Lac was added as Na‐lac and an equimolar of MS was omitted to maintain osmolarity and constant Na + and Cl − concentrations. Elevation of [K + ] o to 11 mM reduced M‐wave area and force in muscles stimulated via the nerve to 16 and 17% of control values at 4 mM [K + ] o , respectively. Subsequent addition of 20 mM lac recovered M‐wave area and force to 53 and 70% of control values at 4 mM [K + ] o , respectively. In similar experiments with field stimulation, the force recovered from 29 to 88% of control values at 4 mM [K + ] o . 20 mM lac reduced pH i from 7.19 to 7.06 within 40 sec, followed by a recovery of pH i to 7.21 over the next 45 min. We have previously shown that a reduction in pH i can lead to force recovery in K + depressed muscles, it was of interest to estimate to what extend the reduction in pH i after addition of lac was responsible for the recovery of force. The reduction in pH i induced by lac was mimicked by increasing CO 2 to 7%, which reduced pH i from 7.19 to 7.04. This pH i reduction, however, only produced a negligible and insignificant recovery of force in muscles depressed by elevated [K + ] o . Conclusion: In K + depressed muscles, a physiological increase in intracellular lac restores excitability and force. Danish Medical Research Council: j.nr. 271‐05‐0304