Increased Excitability of Acidified Skeletal Muscle: Role of Chloride Conductance

Generation of the action potentials (AP) necessary to activate skeletal muscle fibers requires that inward membrane currents exceed outward currents and thereby depolarize the fibers to the voltage threshold for AP generation. Excitability therefore depends on both excitatory Na currents and inhibitory K and Cl currents. During intensive exercise, active muscle loses K and extracellular K ([K ] o ) increases. Since high [K ] o leads to depolarization and ensuing inactivation of voltage-gated Na channels and loss of excitability in isolated muscles, exercise-induced loss of K is likely to reduce muscle excitability and thereby contribute to muscle fatigue in vivo. Intensive exercise, however, also leads to muscle acidification, which recently was shown to recover excitability in isolated K -depressed muscles of the rat. Here we show that in rat soleus muscles at 11 mM K , the almost complete recovery of compound action potentials and force with muscle acidification (CO 2 changed from 5 to 24%) was associated with reduced chloride conductance (1731 151 to 938 64 S/cm 2 , P 0.01) but not with changes in potassium conductance (405 20 to 455 30 S/cm 2 , P 0.16). Furthermore, acidification reduced the rheobase current by 26% at 4 mM K and increased the number of excitable fibers at elevated [K ] o . At 11 mM K and normal pH, a recovery of excitability and force similar to the observations with muscle acidification could be induced by reducing extracellular Cl or by blocking the major muscle Cl channel, ClC-1, with 30 M 9-AC. It is concluded that recovery of excitability in K -depressed muscles induced by muscle acidification is related to reduction in the inhibitory Cl currents, possibly through inhibition of ClC-1 channels, and acidosis thereby reduces the Na current needed to generate and propagate an AP. Thus short term regulation of Cl channels is important for maintenance of excitability in working muscle. key words: lactic acid • muscle fatigue • action potentials • Na channels • Cl channels I N T R O D U C T I O N The activation of a mammalian skeletal muscle fiber requires that action potentials can be generated at the neuromuscular junction and subsequently propagate along the surface membrane and deep into the muscle fiber along the t-tubular system (t-system). To initiate an action potential, the inward membrane current must be sufficient to bring the membrane potential (V m ) above the voltage threshold for action potential generation. Because the change in membrane potential in response to excitatory currents depends on the passive membrane properties and since the chloride conductance (G Cl ) accounts for around 80% of the total membrane conductance at rest, action potential generation and propagation strongly depend on the balance between the excitatory Na currents and the inhibitory or shunting Cl currents. The importance of this balance for muscle excitability is perhaps best illustrated by the dramatic hyperexcitability observed in myotonia congenita, a disease that is caused by a mutation in the gene coding for the major muscle Cl channel ClC-1 (Koch et al., 1992). Moreover, myotonia congenita is often treated with Na channel blockers (LehmannHorn and Jurkat-Rott, 1999), again indicating that the initiation of action potentials in skeletal muscles depend on the interplay between excitatory Na currents and shunting Cl currents. During exercise, active muscle lose K and extracellular K ([K ] o ) has been reported to increase from around 4 to 10 mM in plasma, and even higher levels of [K ] o have been reported in the interstitium (Sréter, 1962; Hnik et al., 1976; Hermansen et al., 1984; Hallen et al., 1994; Green et al., 2000; Juel et al., 2000; Sejersted and Sjøgaard, 2000). Exposure of isolated rat muscles to elevated [K ] o corresponding to the levels measured in vivo starts a chain of events that primarily affects the excitatory Na currents. At elevated [K ] o , fibers become depolarized, Na channels become slow inactivated (Ruff, 1996), the amplitude of single action potentials becomes reduced (Rich and Pinter, 2001), and the amplitude and integrated area of compound action potentials (M-waves) decrease. Based on such findings, elevated [K ] o leading to reduced muscle excitability through loss of excitatory Na currents is believed to be a component in muscle fatigue in working muscle (Sejersted and Sjøgaard, 2000). Correspondence to Thomas Holm Pedersen: [email protected] Abbreviations used in this paper: 9-AC, 9-anthracene-carboxylic acid; t-system, t-tubular system. on Jne 4, 2017 D ow nladed fom Published January 31, 2005