We have examined the binding of (-)[125]-cyanopindolol ((-)[125I]-CYP) to membranes prepared from uterus and lung of dioestrous and post-partum (1-6 days) guinea-pigs. 2. The densities of beta-adrenoceptor binding sites in circular and longitudinal myometrium from dioestrous animals were similar, and approximately one-eight of those in the lung. Ascorbate and ethylenediaminetetraacetic acid in the incubation medium did not affect binding. 3. The numbers and affinities of beta-adrenoceptor binding sites in both myometrial layers and in lung parenchyma from post-partum animals were similar to those in corresponding tissues from dioestrous animals. 4. The distribution of beta-adrenoceptor binding sites in the post-partum uterus was examined using receptor autoradiography. Binding to circular and longitudinal muscle layers and to the endometrium was inhibited by (-)-propranolol (1 mumol/l), by the beta 2-adrenoceptor selective antagonist ICI 118,551 (70 nmol/l), but not by the beta 1-adrenoceptor selective antagonist CGP 20712A (100 nmol/l), indicating that the beta-adrenoceptor present was of the beta 2-subtype. 5. The ability of isoprenaline to compete for (--)[125]-CYP binding sites in uterine membranes from post-partum animals was approximately twice that in corresponding preparations from dioestrous animals. 6. Changes in the numbers or affinity of beta 2-adrenoceptors cannot account for the marked and selective enhancement of the actions of sympathomimetic amines at beta-adrenoceptors previously observed in longitudinal myometrium taken from post-partum guinea-pigs. It is suggested that enhancement of later steps in the chain of events between beta-adrenoceptor occupancy and uterine relaxation, and/or a decrease in the contribution of alpha-adrenoceptors to the net effect of the amine might provide alternative explanations.
