The alpha-2 adrenoceptor radioligand tritiated yohimbine was used to determine the location of pre- versus postsynaptic alpha-2 adrenoceptors in canine nasal mucosa. Using Scatchard analysis, the maximal binding capacity (Bmax) and an apparent equilibrium dissociation constant (Kd) of the alpha-2 adrenoceptor were measured in seven normal dogs and in seven dogs subjected to a unilateral superior cervical ganglionectomy. The results showed that most alpha-2 adrenoceptors in the canine nasal mucosa were located at postsynaptic membranes and very few were located at presynaptic membranes. After superior cervical ganglionectomy, there were no significant changes of the affinity (Kd) and number (Bmax) of the alpha-2 adrenoceptors in the mucosa. These results suggest that the postsynaptic alpha-2 receptors are located extrasynaptically.
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