Comparison of angiotensin receptors in isolated smooth muscle tissues by photoaffinity labelling

The isolated rat uterus and rabbit aorta were photolabelled with [Azidobenzoic acid 1, isoleucine8]angiotensin II ([AB', Ile8]ANG II) and dose-response curves to angiotensins II and III were determined before and after the irreversible labelling procedure. The data obtained were used to calculate affinities, efficacies and 'spare' receptors for angiotensins II and III. In the rat uterus angiotension II was the higher affinity ligand, whereas in the rabbit aorta angiotensins II and III had similar affinities. 'Spare' receptors were present for both ligands in both tissues. In the rat uterus ANG II and ANG III had similar efficacies, whereas in the rabbit aorta ANG II was the more efficacious ligand. Angiotensin receptors in the rabbit aorta, rat uterus, and rat portal vein appear to be substantially different from one another, and point to the possibility of different functional roles for angiotensins II and III in different smooth muscle tissues. For the vascular tissues, tachyphylaxis to angiotensin was more marked after irreversible labelling than before, suggesting that the mechanism of angiotensin tachyphylaxis is receptor deactivation.