Resting membrane potential and intracellular [Na + ] at rest, during fatigue and during recovery in rat soleus muscle fibres in situ.

Large trans-sarcolemmal ionic shifts occur with fatiguing exercise or stimulation of isolated muscles. However, it is unknown how resting membrane potential (E M ) and intracellular sodium concentration ([Na + ] i ) change with repeated contractions in living mammals. We investigated (i) whether [Na + ] i (peak, kinetics) can reveal changes of Na + -K + pump activity during brief or fatiguing stimulation and (ii) how resting E M and [Na + ] i change during fatigue and recovery of rat soleus muscle in situ. Muscles of anaesthetised rats were stimulated with brief (10 s) or repeated tetani (60 Hz for 200 ms, every 2 s, for 30 s or 300 s) with isometric force measured. Double-barrelled ion-sensitive microelectrodes were used to quantify resting E M and [Na + ] i . Post-stimulation data were fitted using polynomials and back-extrapolated to time zero recovery. Mean pre-stimulation resting E M (layer 2-7 fibres) was -71 mV (surface fibres were more depolarised), and [Na + ] i was 14 mM. With deeper fibres, 10 s stimulation (2-150 Hz) increased [Na + ] i to 38-46 mM whilst simultaneously causing hyperpolarisations (7.3 mV for 2-90 Hz). Fatiguing stimulation for 30 s or 300 s led to end-stimulation resting E M of -61 to -53 mV, which recovered rapidly (T 1/2 , 8-22 s). Mean end-stimulation [Na + ] i increased to 86-101 mM with both fatigue protocols and the [Na + ] i recovery time-course (T 1/2 , 21-35 s) showed no difference between protocols. These combined findings suggest that brief stimulation hyperpolarises the resting E M , likely via maximum Na + -induced stimulation of the Na + -K + pump. Repeated tetani caused massive depolarisation and elevations of [Na + ] i that together lower force, although they likely interact with other factors to cause fatigue. [Na + ] i recovery kinetics provided no evidence of impaired Na + -K + pump activity with fatigue. KEY POINTS: It is uncertain how resting membrane potential, intracellular sodium concentration ([Na + ] i ), and sodium-potassium (Na + -K + ) pump activity change during repeated muscle contractions in living mammals. For rat soleus muscle fibres in situ, brief tetanic stimulation for 10 s led to raised [Na + ] i , anticipated to evoke maximal Na + -induced stimulation of the Na + -K + pump causing an immediate hyperpolarisation of the sarcolemma. More prolonged stimulation with repeated tetanic contractions causes massive elevations of [Na + ] i , which together with large depolarisations (via K + disturbances) likely reduce force production. These effects occurred without impairment of Na + -K + pump function. Together these findings suggest that rapid activation of the Na + -K + pump occurs with brief stimulation to maintain excitability, whereas more prolonged stimulation causes rundown of the trans-sarcolemmal K + gradient (hence depolarisation) and Na + gradient, which in combination can impair contraction to contribute to fatigue in living mammals.