Endurance training, not acute exercise, differentially alters beta-receptors and cyclase in skeletal fiber types

Abstract

beta-Adrenergic receptor binding characteristics and adenylate cyclase activity were examined in rat skeletal muscle membranes to determine if acute exercise or endurance training altered beta-receptors or adenylate cyclase activity in different muscle fiber types. Binding characteristics and adenylate cyclase activity were examined in type IIA [red fast-twitch, red vastus (RV)], type IIB [white fast-twitch, white vastus (WV)], and type I [red slow-twitch, soleus (S)] muscles. Acute exercise involved a 20-min run on a treadmill at 20 m/min and did not alter beta-receptor density or adenylate cyclase activity in any of the fiber types examined. Endurance training consisted of a progressive treadmill protocol that involved increasing intensity and duration of exercise for 18 wk. beta-Adrenergic receptor density increased in skeletal muscle fiber types primarily recruited during submaximal training (types I and IIA), whereas nonreceptor-mediated adenylate cyclase activity was altered in the three fiber types. Endurance training significantly increased beta-receptor density in RV by 25% and in S by 19% (P less than 0.05), whereas in WV beta-receptor density was not altered. Basal adenylate cyclase activity in RV was increased approximately 2.5 fold by endurance training. Nonreceptor-mediated adenylate cyclase activity, stimulated by NaF and forskolin, increased by approximately twofold in both RV and WV as a result of endurance training. The data support and extend previous observations to show greater effects of endurance training in types I and IIA fibers with respect to alterations in beta-receptor density and alterations in adenylate cyclase activity in each fiber type. Acute exercise did not alter these parameters either in trained or untrained rats.

Effects of Intermittent Weight-Bearing and Clenbuterol on Disuse Atrophy of Rat Hindlimb Muscles

“…In rat skeletal muscles, the metabolic characteristic (oxidative potential) correlated with the density of β-receptors, and type I fibers have more β-receptors and higher adenylate cyclase activity 47) . If these findings and knowledge are combined with the fact that physical training elevates the number of β-receptors and adenylate cyclase activity 48) , it seems likely that a combination of IWB and Cb medication is useful in suppressing atrophy of the SOL (in which type I fibers are predominant) caused by the absence of weight-bearing. It has been reported that the effect of Cb in suppressing atrophy is higher on denervated muscles than in normally innervated muscles 49)50) .…”

cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration

“…First, epinephrine is released into circulation during exercise (67), and intramuscular cAMP increases within minutes of treadmill running (72). Second, ␤-AR density is highest in oxidative muscles and correlates with oxidative capacity in mixed or intermediate fiber types (62,156,240); AC activity is also increased by endurance training in rats (28). Third, in cultured muscle cells, agents that induce intracellular cAMP activate oxidative enzymes, some at the level of expression (66,136).…”

“…Interestingly, both AC2 and AC7 mRNAs are enriched in fast myofibers (87,232), so it is possible that these fibers have more sustained cAMP production in a chronic treatment setting. However, in rats, AC and PKA activity are both higher in slow-twitch soleus than in fast-twitch extensor digitorum longus muscles (28,104,240), and AC activity increases more after exercise training in oxidative muscles (28). Finally, although CREB stimulates genes involved in oxidative metabolism (242), it is possible that cAMP-induced transcriptional responses are dampened by autoregulatory feedback loops after prolonged ␤-AR activation (236).…”

Muscle fuel selection: effect of exercise and training

“…Since P-receptor mechanisms regulate skeletal muscle TG hydrolysis (Oscai et al 1990), it is possible that an increased @-receptor density may at least partially oppose the lower sympatho-adrenal activation in the trained state. However, to date, increased density of P-receptors has been found in response to training only in the rat (Williams et al 1984;Buckenmeyer et al 1990). …”