Effect of a heptapeptide angiotensin analog on aldosterone secretion in the rabbit

We examined the effects of an angiotensin III (AIII) analog, isoleucine-7 AIII (Ile7-AIII), on the steroidogenic and pressor responses to angiotensin II (AII) and AIII. AII or AIII (25 ng/kg/min) were infused alone or superimposed on an infusion of Ile7-AIII (100 ng/kg/min) in conscious male New Zealand rabbits. AII and AIII induced comparable increases in plasma aldosterone concentration, but AII exhibited significantly greater pressor effect. Ile7-AIII infusion resulted in significant inhibition of AII- but not AIII-induced steroidogenesis. Despite inhibition of aldosterone production, Ile7-AIII failed to block the pressor or renin-suppressing effect of AII. Inhibition of the steroidogenic effect of AII, but not of AII, by Ile7-AIII may be taken as evidence that adrenal stimulation by AII is direct and is not mediated by in vivo conversion to the heptapeptide. The ability of the heptapeptide analog to block aldosterone stimulation by the octapeptide AII suggests that adrenal receptors may, however, have a greater affinity for the heptapeptide.