Atrial natriuretic peptides: the role of phenylalanine on biological activity

Previous studies have shown that atrial natriuretic peptides (ANPs) inhibit the secretion of aldosterone by isolated rat adrenal glomerulosa cells stimulated by angiotensin II, ACTH, and potassium. Structure-function studies have concentrated on the significance of the C- and N-terminal chains for the biological activity of the peptide. We investigated the role of phenylalanine at positions 8 and 26 by using [Ala8]human (h) ANP or [Ala26]hANP analogs to inhibit potassium-stimulated aldosterone secretion in granulosa cells. hANP-(1-28) inhibited potassium-stimulated aldosterone secretion with an IC50 of 0.48 nM. Synthetic [Ala26]hANP inhibited the aldosterone response to potassium with an inhibitory curve relative to hANP-(1-28) (rIC50) of 6.0 nM, which was significantly greater than that for hANP (P less than 0.001). Synthetic [Ala8]hANP was markedly less effective as an inhibitor, with an estimated rIC50 of 3.0 microM (P less than 0.0001). To determine whether the analogs act as competitive antagonists to hANP-(1-28), experiments were performed in which a fixed concentration (0.1 microM) of the analog was incubated in the presence of increasing concentrations of hANP-(1-28). When hANP-(1-28) was incubated with [Ala8]hANP, the rIC50 (0.2 nM) was significantly less than that for hANP-(1-28) alone (P less than 0.02). When hANP-(1-28) was incubated with [Ala26]hANP, the rIC50 was 0.1 nM. In summary, [Ala8]hANP and [Ala26]hANP were significantly less potent than hANP-(1-28) as inhibitors of aldosterone production from granulosa cells. Both analogs shifted the hANP-(1-28) dose-response curve to the left. Neither analog functioned as a competitive antagonist to hANP-(1-28). Our results indicate that the hydrophobic phenyl groups at these two positions are required for full biological potency of ANP as an inhibitor of aldosterone production.