Abstract

Somatostatin (SRIF) is the main inhibitory peptide regulating growth hormone (GH) secretion. It has been difficult to establish the role of endogenous SRIF release in the absence of pure SRIF antagonists. Although several SRIF antagonists have recently been described, none have been shown to possess in vivo activity in the absence of added SRIF. Here, an SRIF antagonist with no detectable agonist activity has been identified from a synthetic combinatorial hexapeptide library containing 6.4 × 10⁷ unique peptides. Each peptide in the library is amino-terminally acetylated and carboxyl-terminally amidated and consists entirely ofd-amino acids. A SRIF-responsive yeast growth assay was used as a primary screening tool, and cAMP accumulation, competitive binding, and microphysiometry also were used to confirm and further characterize SRIF antagonist activity. The hexapeptide library was screened in stepwise iterative fashion to identify AC-178,335, a pure SRIF antagonist of the sequence Ac-hfirwf-NH₂. Thisd-hexapeptide bound SRIF receptor type 2 with an affinity constant (K _i) of 172 ± 12 nm, blocked SRIF inhibition of adenylate cyclase in vitro (IC₅₀ = 5.1 ± 1.4 μm), and induced GH release when given alone (50 μg intravenously) to anesthetized rats with or without pretreatment with a long-acting SRIF agonist.