In pursuit of the alpha-adrenoceptor: a fine-structural and electron autoradiographic study using 3H-phenoxybenzamine and smooth muscle from the cat and guinea-pig.

1. Electron microscopical evaluation of sets of serial fine sections of pancreatic arteriolar muscle of the cat showed no significant difference between the number of pinocytotic vesicles per unit length of smooth muscle cell membrane in ;synaptic' and ;non-synaptic' regions.2. With high resolution autoradiography, (3)H-phenoxybenzamine was found to be distributed over the cytoplasm and nuclei of smooth muscle cell profiles of guinea-pig vas deferens, although no localization in relation to discrete morphological organelles was evident.3. ;Specific protection' of alpha-adrenoceptors using 606 muM noradrenaline resulted in a significant diminution in the binding of (3)H-phenoxybenzamine as evidenced by a decrease in silver grain concentrations over smooth muscle cell profiles in autoradiographs.4. The significance of these findings in relation to the site and distribution of alpha-adrenoceptors is discussed.