Change of activity in rabbit aorta of three analogs of angiotensin substituted in nh2-terminal position.

Activity of Asp-1 -beta-amide-Val-5 -angiotensin II (A II), Sar-1 -Ile-5 -angiotensin II (SA II) and (N,N-dimethyl) Gly-1 -Ile-5 angiotensin II (DMGA II ) was studied in the isolated continuously superfused rabbit aortic strips. The myotropic activity of SA II was found to be 20 times greater than that of A II and DMGA II. A rapid tachyphylaxis occurred to SA II but not to A II and DMGA II. A II and DMGA II but not SA II significantly decreased in their myotropic activities when superfused over the rabbit aortic strips. The observed decrease in activities of A II and DMGA II is probably due to their metabolic degradation in the aortic tissue. The enzymes responsible for this degradation are probably aminopeptidases, as evidence by the fact the SA II, as aminopeptidase-resistant analog of A II, does not lose its activity when superfused over aortic strips.