Effect of covalent dimer conjugates of angiotensin II on receptor affinity and activity in vitro

Angiotensin II [1-8 or 2-8] analogues and [4-8] fragments were dimerized through the amino- or carboxy-terminal groups in order to try to increase their potency as reported for other hormones. The binding affinity to the angiotensin II receptor subtypes A (A IIA) and B (A IIB) was tested and compared to the potency in rabbit aortic ring. The [2-8] dimers coupled through the N-terminus show no significant change in potency in aortic ring. The [4-8] fragments coupled through the N-terminus are inactive in the ring. They have however a significantly increased affinity for the A IIA receptor, the specific function of which has not yet been reported. When angiotensin II analogues or fragments are coupled through the C-terminus, there was a significant drop in affinity and potency, confirming the importance of the free carboxyl group in position 8 for binding and activity. It is concluded that binding to the A IIB receptor correlates well with the effectiveness in aortic ring. However, in contrast to the beneficial effect reported for a large number of other hormones, dimerization of angiotensin II or its fragments is not accompanied by an increased biological activity in aortic ring.