Interactions of Some Partial Agonists with Beta-Adrenoceptor in the Isolated Taenia Caecum and Tracheal Smooth Muscle of Guinea Pig

Abstract

The indexes for agonistic activity (pD₂ values) and for antagonistic activity (pA₂ values) of some beta-adrenergic partial agonists were estimated in the isolated taenia caecum and tracheal preparation of guinea pig. The pK_i values, negative log of dissociation constants, were also estimated by photoinactivation of the beta-adreneoceptor in the taenia caecum. The pD₂ values of the partial agonists were significantly different from their pA₂ values, but equal to their pK_i values. If the partial agonists interacted with one site in their receptor, the pD₂ values would be equal to the pA₂ values because the partial agonists had little receptor reserve. These results suggest that the properties of the binding site for agonistic action were different from those for antagonistic action.