Potassium-induced potentiation of subtetanic force in rat skeletal muscles: influences of β2-activation, lactic acid, and temperature

Moderate elevations of extracellular K+ concentration ([K+](o)) 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), beta(2)-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 mu M), lactic acid (18.1 mM), 9-anthracene-carboxylic acid (9-AC; 25 mu M), or increased temperature (30-35 degrees C). Elevating [K+](o) 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%, P < 0.001) and subtetanic force ( + 22 +/- 4%, P < 0.001). In contrast, salbutamol reduced subtetanic force (-28 +/- 6%, P < 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+](o). The potentiating effect of elevated [K+](o), was still well maintained at 35 degrees 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+](o), whereas lactic acid, 9-AC, or increased temperature does not exert any further augmentation. However, the potentiating effect of elevated [K+](o) was still maintained in the presence of these, thus emphasizing the positive influence of moderately elevated [K+](o) for contractile performance during exercise.

Automated summary

Elevated extracellular K+ concentration can enhance muscle force, while salbutamol further potentiates force in fast-twitch muscles but not in slow-twitch muscles already potentiated by moderately elevated K+ concentration. Lactic acid, 9-AC, and increased temperature do not exert significant effects on muscles exposed to moderate elevations of K+ concentration.