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

Abstract

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.

Keywords: Na+‐K+‐ATPase; depolarisation; hyperpolarisation; maximal sodium transport; sodium–potassium pump.