Different beta-adrenergic receptor density in different rat skeletal muscle fibre types

The effects of adrenaline on skeletal muscle differ between fibre types. The aim of the present study was to investigate the beta-adrenoceptor density, affinity and subtype in rat skeletal muscles with different fibre type composition. beta-Adrenoceptors were determined in cryostat sections to avoid methodological problems with variable recovery, using the non-selective beta-adrenoceptor ligand [3H]CGP-12177 and beta 1- and beta 2-selective cold ligands CGP 20712A and ICI 118,551. In the presence of protease inhibitors [3H]CGP-12177 binding was stable, saturable, reversible, and displaceable. Scatchard analysis of binding saturation data was compatible with a single class of specific binding sites. Binding site density (Bmax) was higher (P < 0.02) in adult soleus (9.38 +/- 1.13 fmol x mg protein-1) than in adult extensor digitorum longus (4.74 +/- 0.39 fmol x mg protein-1), whereas the dissociation constants (Kd), 0.37 +/- 0.05 and 0.31 +/- 0.04 nM for soleus and extensor digitorum longus, respectively, were not significantly different. For young rats (5-6 weeks), Bmax was 11.21 +/- 0.33 and 5.45 +/- 0.11 fmol x mg protein-1 (P < 0.05), and Kd was 0.27 +/- 0.02 and 0.24 +/- 0.04 nM for soleus and epitrochlearis, respectively. These results correspond to a receptor density of 2 and 1 pmol x g w.wt.-1 in muscles containing.mainly type I and type II fibres, respectively. Displacement studies with CGP 20712A and ICI 118,551 were compatible with mainly beta 2-adrenoceptors, but 7-10% beta 1-adrenoceptors were present in both types of muscle.(ABSTRACT TRUNCATED AT 250 WORDS)