Involvement of threshold level in the contractile responses for some alpha 1-andrenoceptor agonists in the rabbit iris dilators.

1. The relationship between receptor occupancies and contractile responses for some alpha 1-adrenoceptor agonists were investigated in rabbit iris dilator smooth muscles. 2. Noradrenaline acted as a full agonists, while oxymetazoline and xylometazoline behaved as partial agonists with moderately higher intrinsic activity, and tizanidine and clonidine were partial agonists with lower intrinsic activity. 3. The pD2-values of oxymetazoline and xylometazoline were practically equal to the corresponding pKB-values, the negative log of dissociation constant, estimated by the partial irreversible blockade of alpha 1-adrenoceptors with phenoxybenzamine. However, the pD2-values of tizanidine and clonidine were significantly lower than the corresponding pKB-values. 4. The threshold phenomena lay between the receptor occupations and tissue responses, therefore, the pKB-values of partial agonists with lower intrinsic activity were different from their pD2-values. 5. These results suggest that the threshold phenomena in the tissue used may be an important factor in determining the agonist sensitivity.