Effects of a beta-adrenergic agonist on protein turnover in muscle cells in culture

β2-Adrenoceptor agonist fenoterol enhances functional repair of regenerating rat skeletal muscle after injury

β2-Adrenoceptor agonists such as fenoterol are anabolic in skeletal muscle, and because they promote hypertrophy and improve force-producing capacity, they have potential application for enhancing muscle repair after injury. No previous studies have measured the β2-adrenoceptor population in regenerating skeletal muscle or determined whether fenoterol can improve functional recovery in regenerating muscle after myotoxic injury. In the present study, the extensor digitorum longus (EDL) muscle of the right hindlimb of deeply anesthetized rats was injected with bupivacaine hydrochloride, which caused complete degeneration of all muscle fibers. The EDL muscle of the left hindlimb served as the uninjured control. Rats received either fenoterol (1.4 mg·kg-1·day-1) or an equal volume of saline for 2, 7, 14, or 21 days. Radioligand binding assays identified a ∼3.5-fold increase in β2-adrenoceptor density in regenerating muscle at 2 days postinjury. Isometric contractile properties of rat EDL muscles were measured in vitro. At 14 and 21 days postinjury, maximum force production (Po) of injured muscles from fenoterol-treated rats was 19 and 18% greater than from saline-treated rats, respectively, indicating more rapid restoration of function after injury. The increase in Poin fenoterol-treated rats was due to increases in muscle mass, fiber cross-sectional area, and protein content. These findings suggest a physiological role for β2-adrenoceptor-mediated mechanisms in muscle regeneration and show clearly that fenoterol hastens recovery after injury, indicating its potential therapeutic application.

Beta-adrenoceptors and the effect of beta-agonists on protein metabolism in ovine primary muscle cultures

The beta-adrenergic receptors of differentiated ovine muscle cultures derived from either fetal or pre-pubertal lambs were characterized by binding of (+/-)-[3H]CGP-12177, directly to intact cells in monolayer. Fetal muscle cells contained a single class of specific and saturable binding sites which had a dissociation constant (Kd) of 0.38 x 10(-9) M and a binding capacity of 55.2 fmol/micrograms protein. beta-Adrenergic agonists competed for the specific binding sites with a typical beta 2-adrenergic specificity. Satellite muscle cells derived from pre-pubertal lambs contained two classes of binding site. The high affinity site had a Kd of 1.02 x 10(-9) M and a binding capacity of 28.4 fmol/micrograms protein and the low affinity site a Kd of 12.1 x 10(-9) M and a binding capacity of 389 fmol/micrograms protein. beta-Adrenergic agonists competed for the specific binding sites with a typical beta 1-adrenergic specificity. The beta-agonist cimaterol had no effect on either protein synthesis or degradation in fetal muscle cells. In cultures derived from satellite cells cimaterol significantly stimulated protein synthesis at concentrations of 10(-8) - 10(-7) M and at 10(-8) - 10(-6) M in the presence of serum. These effects were maintained if 10(-5) M propranolol was added to the incubation media, but were blocked by 10(-6) M isoproterenol. Propranolol and isoproterenol had no stimulatory effects on protein synthesis. Cimaterol also had no detectable effects on protein degradation or the transport of amino acids or glucose. It is concluded that although beta-adrenergic receptors are present in ovine muscle cultures they may not play a role in the anabolic effect of beta-agonists observed in cultured muscle cells.

The effects of beta agonists on muscle cells in culture

Increases in protein synthesis of 12% were found with two myogenic cell lines (L6 and G8-1) on treatment for 6 hr with the beta-adrenergic agonist cimaterol. In L6 cells, propranolol blocked the effect. Protein breakdown measured over 18-24 hr was unchanged. The Kd for cimaterol binding to the L6 beta-receptor was 26 nM which was compatible with its EC50 for the stimulation of protein synthesis (approx 5 nM). Evidence provided with muscle cell lines indicates a direct effect of cimaterol on protein synthesis, which may contribute to muscle accretion in cimaterol-fed animals.