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

Abstract

PURPOSE\nModerate elevations of [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), β2-adrenoceptor activation, and increased temperature would influence the potentiating effect of potassium in slow- and fast-twitch muscle.\n\n\nMETHODS\nIsometric 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 either salbutamol (5 μM), lactic acid (18.1 mM), 9-AC (25 μM) or increased temperature (30 to 35°C).\n\n\nRESULTS\nElevating [K+] from 4 mM to 7 mM (soleus) and 10 mM (EDL) potentiated isometric twitch and subtetanic force while slightly reducing tetanic. 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°C.\n\n\nCONCLUSION\nAddition of salbutamol exerts a further force-potentiating effect in fast-twitch but not in slow-twitch muscles already potentiated by moderately elevated [K+]o, whilst neither lactic acid, 9-AC nor increased temperature exerts 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.