Moderately elevated extracellular [K⁺] potentiates submaximal force and power in skeletal muscle via increased [Ca²⁺]_i during contractions

The extracellular K⁺ concentration ([K⁺]_o) increases during physical exercise. We here studied whether moderately elevated [K⁺]_o may increase force and power output during contractions at in vivo-like subtetanic frequencies and whether such potentiation was associated with increased cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) during contractions. Isolated whole soleus and extensor digitorum longus (EDL) rat muscles were incubated at different levels of [K⁺]_o, and isometric and dynamic contractility were tested at various stimulation frequencies. Furthermore, [Ca²⁺]_i at rest and during contraction was measured along with isometric force in single mouse flexor digitorum brevis (FDB) fibers exposed to elevated [K⁺]_o. Elevating [K⁺]_o from 4 mM up to 8 mM (soleus) and 11 mM (EDL) increased isometric force at subtetanic frequencies, 2-15 Hz in soleus and up to 50 Hz in EDL, while inhibition was seen at tetanic frequency in both muscle types. Elevating [K⁺]_o also increased peak power of dynamic subtetanic contractions, with potentiation being more pronounced in EDL than in soleus muscles. The force-potentiating effect of elevated [K⁺]_o was transient in FDB single fibers, reaching peak after ~4 and 2.5 min in 9 and 11 mM [K⁺]_o, respectively. At the time of peak potentiation, force and [Ca²⁺]_i during 15-Hz contractions were significantly increased, whereas force was slightly decreased and [Ca²⁺]_i unchanged during 50-Hz contractions. Moderate elevation of [K⁺]_o can transiently potentiate force and power during contractions at subtetanic frequencies, which can be explained by a higher [Ca²⁺]_i during contractions.