Isoproterenol enhances force production in mouse glycolytic and oxidative muscle via separate mechanisms

Abstract

Fight or flight is a biologic phenomenon that involves activation of β-adrenoceptors in skeletal muscle. However, how force generation is enhanced through adrenergic activation in different muscle types is not fully understood. We studied the effects of isoproterenol (ISO, β-receptor agonist) on force generation and energy metabolism in isolated mouse soleus (SOL, oxidative) and extensor digitorum longus (EDL, glycolytic) muscles. Muscles were stimulated with isometric tetanic contractions and analyzed for metabolites and phosphorylase activity. Under conditions of maximal force production, ISO enhanced force generation markedly more in SOL (22%) than in EDL (8%). Similarly, during a prolonged tetanic contraction (30 s for SOL and 10 s for EDL), ISO-enhanced the force × time integral more in SOL (25%) than in EDL (3%). ISO induced marked activation of phosphorylase in both muscles in the basal state, which was associated with glycogenolysis (less in SOL than in EDL), and in EDL only, a significant decrease (16%) in inorganic phosphate (P_i). ATP turnover during sustained contractions (1 s EDL, 5 s SOL) was not affected by ISO in EDL, but essentially doubled in SOL. Under conditions of maximal stimulation, ISO has a minor effect on force generation in EDL that is associated with a decrease in P_i, whereas ISO has a marked effect on force generation in SOL that is associated with an increase in ATP turnover. Thus, phosphorylase functions as a phosphate trap in ISO-mediated force enhancement in EDL and as a catalyzer of ATP supply in SOL.

Keywords: Fatigue; Force; Isoproterenol; Metabolites; Muscle; Phosphorylase.

Fig. 1

Scheme for Protocol 1. After setting optimal length (L_O), EDL muscles equilibrated for 20 min, followed by baseline force frequency and a prolonged (continuous) contraction. Thereafter, ISO (or DMSO) was administered for 30 min before force frequency and continuous contraction was repeated. The scheme looks the same for soleus muscles with the exception that after setting L_o, equilibration was for 25 min

Fig. 2

Isoproterenol enhances force generation of soleus and extensor digitorum longus muscles. Force frequency curves were performed at baseline (○, □) and 30 min post-treatment for control (●) and isoproterenol (■) groups in soleus (SOL) (a, b) and extensor digitorum longus (EDL) (c−f), respectively. Forces are substantially higher than baseline in the presence of ISO in SOL (b). After baseline, a prolonged contraction was performed in each muscle (see Fig. 3), which caused a rundown effect in the EDL muscle with CON values significantly decreasing from baseline (c). Consequently, there were only minor increases in force in the presence of ISO (d). Force frequency curves were therefore performed in another series of EDL muscles without continuous contraction after baseline, CON (e) and ISO (f). In the latter case, forces are clearly higher than baseline in the presence of ISO (f). The corresponding average percentage effect of ISO on force is shown for SOL (g) and EDL (h) muscles. h is derived from results from d and f, with adjustments for rundown in d. Data are expressed as mean ± SE for 5 (e−f), 8 (a−d and g), or 13 (h) muscles. *p < 0.05; **p < 0.01; ***p < 0.001 vs. corresponding treatment by paired t test

Fig. 3

Representative force recordings for soleus and extensor digitorum longus muscles during maximal stimulation frequencies. Forces are shown for a given muscle before (solid line) and after (dashed line) treatment. Soleus muscles were stimulated at 100 Hz before and after exposure to diluent (a) or ISO (b). Extensor digitorum longus (EDL) muscles were stimulated at 150 Hz before and after exposure to diluent (c) or ISO (d). Note that at these frequencies ISO enhanced force markedly more in soleus than in EDL

Fig. 4

Isoproterenol enhances work output during prolonged contractions of soleus and extensor digitorum longus muscles. Prolonged tetanic contractions were performed for 30 s at a frequency of 100 Hz in SOL and 10 s at a frequency of 120 Hz in EDL. Representative traces are shown for baseline and 30 min post-treatment in CON and ISO groups as indicated for SOL (a, b) and EDL (d, e), respectively. Note that in a and d, baseline and CON curves are virtually identical, whereas in panel b the ISO curve is markedly higher throughout in comparison to baseline and in e the ISO curve is slightly higher initially vs. baseline. The effect of ISO on force × time integral was quantified by determining the area under the curve (AUC) of the continuous contractions in SOL (c) and EDL (f) muscles at baseline (B, unfilled bar) and 30 min post-treatment (CON and ISO, filled bars). Data are expressed as mean ± SE for eight muscles. *p < 0.05; ***p < 0.001 vs. corresponding baseline values by paired t test

Fig. 5

Isoproterenol activates phosphorylase and inactivates glycogen synthase in soleus and extensor digitorum longus muscles. Phosphorylase (Phos) and glycogen synthase (GS) fractional activities were determined in soleus (SOL, a and c) and extensor digitorum longus muscles (EDL, b and d). Treatments consisted of basal control (BC), basal isoproterenol (BI), stimulated control (SC), and stimulated isoproterenol (SI) in the absence (−) or presence (+) of 3 mM sodium cyanide. SOL muscles were stimulated for 5 s at 100 Hz and EDL muscles for 1 s at 120 Hz. Data are expressed as mean ± SE for 5–7 muscles (numbers vary as sufficient extract was not always available). *p < 0.05; **p < 0.01; ***p < 0.001 vs. corresponding baseline values by paired t test