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MUSCLE CELL BIOLOGY AND CELL MOTILITY

Changes of action potentials and force at lowered [Na⁺]_o in mouse skeletal muscle: implications for fatigue

¹Department of Physiology, School of Medicine, and ²Bioengineering Institute, University of Auckland, and ³Division of Sport and Recreation, Auckland University of Technology, Auckland 1020, New Zealand; and ⁴Department of Cellular and Molecular Medicine, Neuromuscular Research Center, University of Ottawa, Ottawa, Ontario, Canada K1H 8MS

Submitted 30 August 2002 ; accepted in final form 21 June 2003

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.

sodium gradient; muscle contraction; action potential train; extensor digitorum longus; soleus

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