Excitation-induced force recovery in potassium-inhibited rat soleus muscle

Abstract

1. Excitation markedly stimulates the Na+-K+ pump in skeletal muscle. The effect of this stimulation on contractility was examined in rat soleus muscles exposed to high extracellular K+ concentration ([K+]o). 2. At a [K+]o of 10 mM, tetanic force declined to 58 % of the force in standard buffer with 5.9 mM K+. Subsequent direct stimulation of the muscle at 1 min intervals with 30 Hz pulse trains of 2 s duration induced a 97 % recovery of force within 14 min. Force recovery could also be elicited by stimulation via the nerve. In muscles exposed to 12.5 mM K+, 30 Hz pulse trains of 2 s duration at 1 min intervals induced a recovery of force from 16 +/- 2 to 62 +/- 4% of the initial control force at a [K+]o of 5.9 mM. 3. The recovery of force was associated with a decrease in intracellular Na+ and was blocked by ouabain. This indicates that the force recovery was secondary to activation of the Na+-K+ pump. 4. Excitation stimulates the release of calcitonin gene-related peptide (CGRP) from nerves in the muscle. Since CGRP stimulates the Na+-K+ pump, this may contribute to the excitation-induced force recovery. Indeed, reducing CGRP content by capsaicin pre-treatment or prior denervation prevented both the excitation-induced force recovery and the drop in intracellular Na+. 5. The data suggest that activation of the Na+-K+ pump in contracting muscles counterbalances the depressing effect of reductions in the chemical gradients for Na+ and K+ on excitability.

Figure 1. Effect of direct electrical stimulation on isometric force development in muscles at a [K⁺]_o of 5.9 and 10 mm

Isometric tetanic contractions were elicited by 30 Hz pulse trains of 2 s duration (1 ms pulses, 12 V). After 30 min incubation in the standard buffer containing 5.9 mm K⁺, [K⁺]_o was either increased to 10 mm (○), or maintained at 5.9 mm (contralateral control muscles, •). Pulse trains were applied as indicated by data points. From time 120 min, the pulse trains were applied at 1 min intervals. Each point represents the mean of observations on five muscles with vertical bars denoting s.e.m. For sake of clarity, s.e.m. is only shown once every 10 min.

Figure 2. Effect of indirect electrical stimulation on isometric force development in muscles at a [K⁺]_o of 5.9 and 10 mm

Experimental conditions as described in the legend to Fig. 1, except that a pulse duration of 0.02 ms was used from time 30 min to time 180 min as indicated. After 60 min incubation in the standard buffer containing 5.9 mm K⁺, [K⁺]_o was either increased to 10 mm (○), or maintained at 5.9 mm (contralateral control muscles, •). Tubocurarine (10⁻⁵ m) was added to the buffers at time 170 min. Each point represents the mean of observations on five muscles with vertical bars denoting s.e.m. For sake of clarity, s.e.m. is only shown once every 10 min.

Figure 3. Effects of CGRP and of shortening the intervals between 30 Hz pulse trains on isometric force development in muscles at a [K⁺]_o of 12.5 mm

Isometric tetanic contractions were elicited by 30 Hz pulse trains of 2 s duration (1 ms pulses, 12 V) and were applied every 10 min or every 1 min as indicated by data points. After 30 min at a [K⁺]_o of 5.9 mm, [K⁺]_o was increased to 12.5 mm for all muscles as indicated by bar. ○, muscles undergoing two periods with pulse trains every 1 min. After the first period with stimulation every 1 min, the muscles were washed three times in buffer containing 12.5 mm K⁺. CGRP (10⁻⁷ m) was added after 410 min as indicated (n = 7). ▵, muscles undergoing the same experimental protocol but with CGRP added after 70 min of stimulation with pulse trains every 1 min. For sake of clarity only the last 60 min of the experiment are shown (n = 4). The data points represent the means with vertical bars denoting s.e.m. For sake of clarity, s.e.m. is only shown once every 10 min.

Figure 4. Effect of capsaicin on excitation-induced force recovery in muscles at a [K⁺]_o of 12.5 mm

Experimental conditions as described in legend to Fig. 3. ○, capsaicin (5 × 10⁻⁶ m) added after 100 min and washed away after 170 min as indicated. •, controls not treated with capsaicin. CGRP (10⁻⁷ m) was added to both groups of muscles after 290 min. Each point represents the mean of observations on four muscles with vertical bars denoting s.e.m. For sake of clarity s.e.m. is only shown once every 10 min.

Figure 5. Effect of denervation on excitation-induced force recovery in muscles at a [K⁺]_o of 12.5 mm

Experimental conditions as described in the legend to Fig. 3. Rats weighing 50–60 g were anaesthetized and denervated at the sciatic level on one side. After 7 days, muscles were prepared for measurement of force development. ○, denervated muscles. •, contralateral controls. Each point represents the mean of observations on four muscles with vertical bars denoting s.e.m. For sake of clarity, s.e.m. is only shown once every 10 min.