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

The β-adrenergic receptors of differentiated ovine muscle cultures derived from either fetal or pre-pubertal lambs were characterized by binding of (±)-[ 3 H] 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 ( K d of 0.38×10 −9 M and a binding capacity of 55.2 fmol/μg protein. β-Adrenergic agonists competed for the specific binding sites with a typical β 2-adrenergic specificity. Satellite muscle cells derived from pre-pubertal lambs contained two classes binding site. The high affinity site had a K d of 1.02 × 10 −9 M and a binding capacity of 28.4 fmol/μg protein and the low affinity site a K d of 12.1 × 10 −9 M and a binding capacity of 389 fmol/μg protein. β-Adrenergic agonists competed for the specific binding sites with a typical β 1-adrenergic specificity. The β-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 β-adrenergic receptors are present in ovine muscle cultures they may not play a role in the anabolic effect of β-agonists observed in cultured muscle cells.