Reply from Torben Clausen and Ole Bækgaard Nielsen

Abstract

We agree that muscle fatigue may have many causes depending on the kind of exercise employed. In favour of a contribution to fatigue from elevated extracellular K+ ([K+]o) leading to a reduction in muscle excitability, several studies on human subjects have demonstrated a peripheral transmission failure that correlates with the fatigue development during voluntary contractions (Kalmar & Cafarelli, 2004). Our study published in The Journal of Physiology (Clausen & Nielsen, 2007) was prompted by a paper indicating that buffer containing 10 mm K+ caused no impairment of contractile endurance during continuous or repeated tetanic stimulation of mouse soleus at 50 or 70 Hz (Zhang et al. 2006). We therefore used a comparable frequency (60 Hz) and found that the lack of response to 10 mm K+ was likely to be due to Na+,K+-pump stimulation elicited by the use of 20 min of repeated stimulation every 2 min prior to the exposure to 50–70 Hz stimulation at 10 mm K+. We could demonstrate that when the muscles were instead allowed to rest for 20 min before the continuous 60 Hz stimulation, 10 mm K+ induced a 7-fold increase in the rate of force decline which could be restored by stimulation of the Na+,K+-pumps. For the sake of consistency and comparability, all our experiments were performed using 60 Hz. We agree that in vivo, continuous 60 Hz stimulation is not likely to last many seconds. However, we had previously shown that using single twitches or short pulse trains at more physiological frequencies (12–30 Hz), elevated [K+]o (10–12.5 mm) induced a marked inhibition of contractile force, which could be recovered by Na+,K+-pump stimulation (Clausen et al. 1993; Clausen, 2003). What happens when continuous stimulation in vitro is applied at lower frequencies? In rat soleus we find that 10 mm K+ increases the rate of force decline as measured over the first 120 s of 20 Hz stimulation by 135% (from 0.17 ± 0.03% s−1 to 0.40 ± 0.03% s−1, N = 14, P < 0.001). What happens in vivo? When stimulated indirectly in vivo for 4 min at 5 Hz, Sreter (1963) found that the red fibres of rat gastrocnemius showed a 14% decrease in [K+]i and an 81% increase in [Na+]i and soleus a 47% increase in [Na+]i. Others have found that in rat extensor digitorum longus muscle, 15 min of indirect 5 Hz stimulation caused about 100% increase in [Na+]i and 30% decrease in [K+]i (Nagaoka et al. 1994). These results demonstrate that large disturbances in the chemical gradients for K+ also take place in vivo, which is in keeping with the observation that voluntary biking exercise, which is assumed to be associated with an average stimulation frequency of around 6 Hz, leads to a net loss of K+ from human muscle (Sjogaard, 1990; Hallen et al. 1994). We agree that isolated muscles without circulation may develop an anoxic core during contractions (Murphy & Clausen, 2007). As pointed out in the Letter to the Editor, maximum isometric contractions may often lead to occlusion of circulation and anoxia. It is not so surprising, therefore, that muscles have adapted to maintain function under anoxic conditions, probably by glycolysis. Moreover, there is evidence that the Na+,K+-pumps can utilize ATP generated by glycolysis (Dutka & Lamb, 2007) and thereby maintain some excitability during periods of anoxia. We showed that supressing glycolysis by loading soleus muscles with 2-deoxyglucose caused rapid and marked loss of contractile endurance (Murphy & Clausen, 2007). It is not possible to maintain oxygenation in the isolated muscle preparation without circulation, but studies on human subjects show that peripheral transmission failure may also occur in muscles with blood circulation (Kalmar & Cafarelli, 2004). In conclusion, exposure to the extracellular K+ levels likely to occur in skeletal muscles during intense exercise leads to loss of excitability and impairment of contractile endurance. This is faster in onset and more pronounced at high frequencies (60 Hz) than at low frequencies of stimulation. The effect of elevated extracellular K+ on the function of contracting muscle depends, however, on the activation of protective mechanisms, primarily stimulation of the Na+,K+-pumps and inhibition of the Cl− channels (Pedersen et al. 2005).