[Use of specific angiotensin inhibitors in the study of hormone-receptor interaction]

The specificity of receptors participating in the interaction with angiotensin II (AT 1--8) fragments, e. g. N-terminal tri-(AT 1--3), C-terminal penta-(AT 4--8), and middle tetrapeptide (at 3--6), was studied in experiments on rat ascending colon which were performed to investigate the influence of a specific angiotensin antagonist, /1-hydantoic acid, 5-valine, 8-alanine/-angiotensin (HAAT 1--8), on myotropic effects of these fragments as well as the influence of the fragments on the myotropic activity of the whole hormone molecule. Competitive antagonism was found between HAAT 1--8 and AT 4--8 and AT 3--6 and between these fragments and AT 1--8. No antagonisma was revealed between AT 1--3 and HAAT 1--8, and between AT 1--3 and AT 1--8. It is assumed that the fragments AT 4--8 and AT 3--6 interact at the level of angiotensin receptors, and that of AT 1--3 at the level of non-specific receptors. A new model for the structural and functional organization of angiotensin is proposed.