Autoradiographic delineation of skeletal muscle α₁-adrenergic receptor distribution

We used light microscopic autoradiography to quantify the distribution of α₁-adrenergic receptors in vessels and muscle fibers of slow-twitch (type I), fast-twitch (types IIa and IIb), and mixed fiber muscles of the rat hindquarter. Frozen cross sections of soleus, vastus lateralis, and gastrocnemius muscles were incubated under equilibrium binding conditions with 10-200 pM [³H]prazosin with or without 10^-5 M phentolamine. Because of the low concentration of bound radioligand, specific binding could not be detected with scintillation spectrometry in whole tissue sections scraped from slides. However, quantitative autoradiographic analysis after extended intervals of emulsion exposure revealed a low but significant level of specific binding in muscle fibers. No difference in α₁-receptor density was observed among types I, IIa, and IIb fibers. Small blood vessels had a much greater α₁-receptor density than muscle fibers. Resistance arterioles (20-100 μm diam) and small arteries (100-500 μm diam) contained 5.8 ± 0.9 and 31.6 ± 7.6 (± SE) times more binding sites per unit section area, respectively, than did surrounding muscle fibers (both P < 0.001). Ratios of specific grain densities in fibers and blood vessels did not vary with radioligand concentration, indicating that observed grain densities reflected differences in receptor concentration rather than radioligand affinity by fiber and vessel receptors. The densities of vascular α₁-receptors did not vary in slow- and fast-twitch muscles, but resistance arterioles were six and eight times more numerous in soleus than in gastrocnemius and vastus muscles, respectively (both P < 0.001). Thus variation in α₁-receptor density in rat hindquarter muscles reflects differences in vascularity rather than fiber type composition.