Interactions of some partial agonists with beta-adrenoceptor in the isolated taenia caecum and tracheal smooth muscle of guinea pig.

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 pK1 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 pK1 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.