Changes of action potentials and force at lowered [Na+]o in mouse skeletal muscle: implications for fatigue.
We examined 1) whether the effects of lowered trans-sarcolemmal Na+ gradient on force differed between nonfatigued fast- and slow-twitch muscles of mice and 2) whether effects on action potentials could explain the decrease of force. The Na+ gradient was reduced by lowering the extracellular [Na+] ([Na+]o). The peak force-[Na+]o relationships for the twitch and tetanus were the same in nonfatigued extensor digitorum longus and soleus muscles: force was maintained over a large range of [Na+]o and then decreased abruptly over a much smaller range. However, fatigue was significantly exacerbated at a lowered [Na+]o that had little effect in nonfatigued soleus muscle. This finding suggests that substantial differences exist in the Na+ effect on force between nonfatigued and fatigued muscle. The reduced contractility in nonfatigued muscles at lowered [Na+]o was largely due to 1) an increased number of inexcitable fibers and threshold for action potentials, 2) a reduction of action potential amplitude, and 3) a reduced capacity to generate action potentials throughout trains.
Cairns
Simeon P
SP
Department of Physiology, School of Medicine, University of Auckland, Auckland 1020, New Zealand. simeon.cairns@aut.ac.nz
Buller
Sarah J
SJ
Loiselle
Denis S
DS
Renaud
Jean-Marc
JM
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D045604:Extracellular Fluid / Q000187:drug effects / Q000502:physiology; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D051379:Mice; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D018763:Muscle Fatigue / Q000187:drug effects / Q000502:physiology*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology / Q000502:physiology*
D012964:Sodium