β‐ADRENOCEPTORS IN CIRCULAR AND LONGITUDINAL MYOMETRIAL MEMBRANES AND IN LUNG MEMBRANES FROM DIOESTROUS AND POST‐PARTUM GUINEA‐PIGS

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Abstract
SUMMARY: 1. We have examined the binding of (—)[125I]‐cyanopindolol ((—)[125I]‐CYP) to membranes prepared from uterus and lung of dioestrous and post‐partum (1–6 days) guinea‐pigs.2. The densities of β‐adrenoceptor binding sites in circular and longitudinal myometrium from dioestrous animals were similar, and approximately one‐eighth of those in the lung. Ascorbate and ethylenediaminetetraacetic acid in the incubation medium did not affect binding.3. The numbers and affinities of β‐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 β‐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 μmol/l), by the β2‐adrenoceptor selective antagonist ICI 118,551 (70 nmol/l), but not by the β1‐adrenoceptor selective antagonist CGP 20712A (100 nmol/l), indicating that the β‐adrenoceptor present was of the β2‐subtype.5. The ability of isoprenaline to compete for (—)[125I]‐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 β2‐adrenoceptors cannot account for the marked and selective enhancement of the actions of sympathomimetic amines at β‐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 β‐adrenoceptor occupancy and uterine relaxation, and/or a decrease in the contribution of α‐adrenoceptors to the net effect of the amine might provide alternative explanations.

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