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) and intracellular sodium concentration ([Na]) change with repeated contractions in living mammals. We investigated (i) whether [Na] (peak, kinetics) can reveal changes of Na-K pump activity during brief or fatiguing stimulation and (ii) how resting E and [Na] 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 and [Na]. Post-stimulation data were fitted using polynomials and back-extrapolated to time zero recovery. Mean pre-stimulation resting E (layer 2-7 fibres) was -71 mV (surface fibres were more depolarised), and [Na] was 14 mM. With deeper fibres, 10 s stimulation (2-150 Hz) increased [Na] 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 of -61 to -53 mV, which recovered rapidly (T, 8-22 s). Mean end-stimulation [Na] increased to 86-101 mM with both fatigue protocols and the [Na] recovery time-course (T, 21-35 s) showed no difference between protocols. These combined findings suggest that brief stimulation hyperpolarises the resting E, likely via maximum Na-induced stimulation of the Na-K pump. Repeated tetani caused massive depolarisation and elevations of [Na] that together lower force, although they likely interact with other factors to cause fatigue. [Na] 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]), 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], 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], 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.