Different β‐Adrenergic Receptor Density in Different Rat Skeletal Muscle Fibre Types

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Abstract
The effects of adrenaline on skeletal muscle differ between fibre types. The aim of the present study was to investigate the β‐adrenoceptor density, affinity and subtype in rat skeletal muscles with different fibre type composition. β‐Adrenoceptors were determined in cryostat sections to avoid methodological problems with variable recovery, using the non‐selective β‐adrenoceptor ligand [3H]CGP‐12177 and β1 and β2‐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–1) than in adult extensor digitorum longus (4.74 ± 0.39 fmolxmg protein–1), whereas the dissociation constants (Kd), 0.37® pL 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 fmolxmg protein–1 (Pd 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 pmolxg 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 β2‐adrenoceptors, but 7–10% β1‐adrenoceptors were present in both types of muscle. In conclusion, the receptor density is twice as high in muscles containing mainly type I muscle fibres compared to muscles containing mainly type II fibres, and this may explain some of the different effects of adrenaline between the two muscle fibre types.