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Effects of extracellular HCO₃^– on fatigue, pH_i, and K⁺ efflux in rat skeletal muscles

¹Institute of Physiology and Biophysics, ²Department of Sport Science, University of Aarhus, Denmark

Submitted 11 January 2007 ; accepted in final form 16 April 2007

Elevated plasma HCO₃^– can improve exercise endurance in humans. This effect has been related to attenuation of the work-induced reduction in muscle pH, which is suggested to improve performance via at least two mechanisms: 1) less inhibition of muscle enzymes and 2) reduced opening of muscle K_ATP channels with less ensuing reduction in excitability. Aiming at determining whether the ergogenic effect of HCO₃^– is related to effects on muscles, we examined the effect of elevating extracellular HCO₃^– from 25 to 40 mM (pH from 7.4 to 7.6) on fatigue, intracellular pH (pH_i), and K⁺ efflux in isolated rat skeletal muscles contracting isometrically. Fatigue induced by 30-Hz stimulation at 30 and 37°C was similar between soleus muscles incubated in high and normal HCO₃^– concentrations. In extensor digitorum longus muscles stimulated at 60 Hz, elevated HCO₃^– did not affect fatigue at 30°C. In soleus muscles, 30-Hz stimulation induced a 0.2 unit reduction in pH_i, as determined by using the pH-sensitive probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. This reduction in pH_i was not affected by elevated HCO₃^–. Estimation of K⁺ efflux using ⁸⁶Rb⁺ showed that elevated HCO₃^– did not affect K⁺ efflux at rest or during contractions. Similarly, other modifications of the intra- and extracellular pH had little effect on K⁺ efflux during contraction. In conclusion, elevated extracellular HCO₃^– had no significant effect on muscle fatigue, pH_i, and K⁺ efflux. These findings indicate that alternative mechanisms must be considered for the ergogenic effect of HCO₃^– observed in integral exercise studies.

muscle fatigue

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