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

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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), β2-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 + ]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 þ ]o 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 + ]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.

OriginalsprogEngelsk
TidsskriftAmerican Journal of Physiology - Cell Physiology
Vol/bind321
Nummer5
Sider (fra-til)C884-C896
Antal sider13
ISSN0363-6143
DOI
StatusUdgivet - nov. 2021

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