Loss of force induced by high extracellular [K+] in rat muscle: effect of temperature, lactic acid and beta2-agonist

Abstract

Loss of K+ from active muscles, leading to increased [K+]o, has been proposed to cause muscle fatigue by reducing excitability. Since exercise increases muscle temperature, we investigated the influence of temperature on muscle [K+]o sensitivity. Intact rat soleus or extensor digitorum longus (EDL) muscles were mounted on force transducers and stimulated electrically to evoke short isometric tetani at regular intervals. In each experiment, control force at 4 mM K+ was initially determined at every temperature used. In soleus muscles at 20 degrees C, 9 mM K+ reduced force to 33 +/- 5 % of control force. Increasing the temperature to 30 degrees C restored force to 89 +/- 5 % of control force. Likewise, at 30 degrees C 11 mM K+ reduced force to 16 +/- 4 % and increasing the temperature to 35 degrees C restored force to 35 +/- 5 %. Similar results were obtained using EDL. The force recovery induced by elevating temperature, reflecting reduced [K+]o sensitivity, was associated with improved excitability assessed from compound action potentials. Force recovery induced by a temperature elevation from 20 to 30 degrees C was associated with hyperpolarization (5 mV), reduced [Na+]i and a 93 % increase in Na+-K+ pump activity. The force recovery was blocked by ouabain. Since intensive exercise leads to lactic acidosis and increased plasma catecholamines, the effect of these two factors was also investigated. At 11 mM K+, force was completely restored by combining temperature elevation (30 to 35 degrees C), L-lactic acid (10 mM) and the beta2-agonist salbutamol (10-5 M). We suggest an exercise scenario where the depressing action of exercise-induced hyperkalaemia is counteracted by elevated muscle temperature, lactic acidosis and catecholamines.

Figure 1. Effect of temperature on tetanic force at 4 and 9 mM K⁺

A, soleus muscles were stimulated every 10 min using 0.2 ms pulses of 12 V at 30 Hz for 2 s. Tetanic force is expressed in newtons (N) and changes in temperature and [K⁺]_o are indicated by bars. •, 4 mM K⁺ throughout, n = 2. ▵, [K⁺]_o changed from 4 to 9 mM K⁺ after 50 min, n = 6; error bars show S.E.M. B, influence of temperature on force depression induced by 9 mM K⁺. Steady tetanic force at points indicated by a-d in A is related to the control force at 4 mM K⁺ at the corresponding temperature; error bars show S.E.M.

Figure 2. Effect of temperature on [K⁺]_o sensitivity of soleus muscles incubated at high [K⁺]_o

Data points represent steady force expressed relative to force at 4 mM K⁺ at the corsreponding temperatures. Each point is the mean of 6 muscles with error bars showing S.E.M. A, muscles incubated at 20 °C (□) and 30 °C (•) with a stimulation frequency of 30 Hz. Letters indicate data from the experiments shown in Fig. 1A. B, muscles incubated at 30 °C (○) and 35 °C (▴) with a stimulation frequency of 90 Hz.

Figure 3. Effect of temperature on tetanic force at 4 and 11 mM K⁺ in rat EDL muscles

Muscles were stimulated every 20 min using 0.2 ms pulses of 12 V at 90 Hz for 0.5 s. •, 4 mM K⁺ throughout, n = 5; error bars show S.E.M. ▵, [K⁺]_o changed from 4 to 11 mM K⁺ after 60 min, n = 6; error bars show S.E.M.

Figure 4. Effect of temperature on M-wave and force in soleus muscles incubated at 4 and 10 mM K⁺ at 20 and 30 °C

Muscles were stimulated through the nerve using a stimulation frequency of 30 Hz, while recording M-waves and force. The top panel shows recordings of M-waves at 4 and 10 mM K⁺ from the same muscle at 20 and 30 °C. The bottom panel shows steady force and M-wave area at 4 and 10 mM K⁺, related to control recordings at 4 mM K⁺ at the corresponding temperature, n = 6; error bars show S.E.M.

Figure 5. Effect of Na⁺-K⁺ pump inhibition on force at 4 and 9 mM K⁺ at 20 and 30 °C

Experimental conditions as in Fig 1A. Ouabain (10⁻⁵ M) was added 15 min prior to temperature elevation as indicated by arrows. •, 4 mM K⁺ throughout. ▵, [K⁺]_o changed from 4 to 9 mM after 50 min. Each point indicates the mean of observations from 6 muscles with error bars showing S.E.M.

Figure 6. Effects of temperature, 10 mM L-lactic acid and 10⁻⁵ M salbutamol on tetanic force at 11 mM K⁺

Soleus muscles were stimulated every 10 min using 0.2 ms pulses of 12 V at 90 Hz for 2 s. •, 4 mM K⁺ throughout; ▪, 11 mM K⁺ with stepwise addition of 10 mM L-lactic acid and 10⁻⁵ M salbutamol after temperature elevation; ▵, 11 mM K⁺ with addition of L-lactic acid and salbutamol occurring simultaneously with temperature elevation. Each point indicates the mean of 6-12 muscles with error bars showing S.E.M.

Figure 7. Effects of combined addition of 10 mM L-lactic acid and 10⁻⁵ M salbutamol on the [K⁺]_o sensitivity of contracting soleus muscles at 35 °C

Data points represent steady force at the indicated [K⁺]_o expressed relative to force at 4 mM K⁺ at 35 °C. ▴, 35 °C; □, 35 °C with 10 mM L-lactic acid; ▵, 35 °C with 10 mM L-lactic acid and 10⁻⁵ M salbutamol. Each point is the mean of 6 muscles with error bars showing S.E.M.