Radioligand binding studies of postsynaptic alpha 1-adrenoceptors in the rabbit iris dilators

Interactions of several alpha 1-adrenoceptor agonists and antagonists were studied in rabbit iris dilator smooth muscles and aortic strips using radioligand binding techniques. The specific binding of [3H]-prazosin to the membrane preparations of both tissues is of high affinity, saturable with a single binding site, has characteristics expected of alpha 1-adrenoceptors. pKi-Values of prazosin and yohimbine and the pKD-value of [3H]-prazosin in the rabbit dilators, however, were significantly lower than those in the aortic strips, while pKi-values of phentolamine, clonidine and norepinephrine in the dilators were nearly equal to those in the aortic strips. Although the efficacy of norepinephrine in the dilators was significantly less than that in the aortic strips, the maximum binding sites for [3H]-prazosin to the membrane preparations from the rabbit dilators were more numerous than those from the aortic strips. Therefore, it is likely that the intrinsic efficacy of norepinephrine in the dilators is less than that in the aortic strips. These results suggest that alpha 1-adrenoceptors in the rabbit dilator smooth muscle and the aortic strips may not be identical and that both selective and nonselective drugs, which act on these receptor sites, may exist.