Potassium-induced potentiation of subtetanic force in rat skeletal muscles: influences of β-activation, lactic acid, and temperature.

Moderate elevations of extracellular K concentration ([K]) occur during exercise and have been shown to potentiate force during contractions elicited with subtetanic frequencies. Here, we investigated whether lactic acid (reduced chloride conductance), β-adrenoceptor activation, and increased temperature would influence the potentiating effect of potassium in slow- and fast-twitch muscles. Isometric contractions were elicited by electrical stimulation at various frequencies in isolated rat soleus and extensor digitorum longus (EDL) muscles incubated at normal (4 mM) or elevated K, in combination with salbutamol (5 μM), lactic acid (18.1 mM), 9-anthracene-carboxylic acid (9-AC; 25 μM), or increased temperature (30-35°C). Elevating [K] from 4 mM to 7 mM (soleus) and 10 mM (EDL) potentiated isometric twitch and subtetanic force while slightly reducing tetanic force. In EDL, salbutamol further augmented twitch force (+27 ± 3%, < 0.001) and subtetanic force (+22 ± 4%, < 0.001). In contrast, salbutamol reduced subtetanic force (-28 ± 6%, < 0.001) in soleus muscles. Lactic acid and 9-AC had no significant effects on isometric force of muscles already exposed to moderate elevations of [K]. The potentiating effect of elevated [K] was still well maintained at 35°C. Addition of salbutamol exerts a further force-potentiating effect in fast-twitch but not in slow-twitch muscles already potentiated by moderately elevated [K], whereas lactic acid, 9-AC, or increased temperature does not exert any further augmentation. However, the potentiating effect of elevated [K] was still maintained in the presence of these, thus emphasizing the positive influence of moderately elevated [K] for contractile performance during exercise.