Affiliation:
1. Research and Development The Nutraceutical Alliance Inc. Guelph Ontario Canada
2. Sports Performance Research Institute New Zealand, School of Sport and Recreation, Faculty of Health and Environmental Sciences Auckland University of Technology Auckland New Zealand
3. Health and Rehabilitation Research Institute, Faculty of Health and Environmental Sciences Auckland University of Technology Auckland New Zealand
4. Institute for Experimental Medical Research University of Oslo and Oslo University Hospital Oslo Norway
Abstract
AbstractLarge trans‐sarcolemmal ionic shifts occur with fatiguing exercise or stimulation of isolated muscles. However, it is unknown how resting membrane potential (EM) 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 EM 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 EM and [Na+]i. Post‐stimulation data were fitted using polynomials and back‐extrapolated to time zero recovery. Mean pre‐stimulation resting EM (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 EM of −61 to −53 mV, which recovered rapidly (T1/2, 8–22 s). Mean end‐stimulation [Na+]i increased to 86–101 mM with both fatigue protocols and the [Na+]i recovery time‐course (T1/2, 21–35 s) showed no difference between protocols. These combined findings suggest that brief stimulation hyperpolarises the resting EM, 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.
imageKey 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.