A LINEAR HEXAPEPTIDE SOMATOSTATIN ANTAGONIST BLOCKS SOMATOSTATIN ACTIVITY IN-VITRO AND INFLUENCES GROWTH-HORMONE RELEASE IN RATS

Somatostatin (SRIF) is the main inhibitory peptide regulating growth h ormone (GH) secretion. It has been difficult to establish the role of endogenous SRIF release in the absence of pure SRIF antagonists. Altho ugh several SRIF antagonists have recently been described, none have b een shown to possess in vivo activity in the absence of added SRIF. He re, an SRIF antagonist with no detectable agonist activity has been id entified from a synthetic combinatorial hexapeptide library containing 6.4 x 10(7) unique peptides. Each peptide in the library is amino-ter minally acetylated and carboxyl-terminally amidated and consists entir ely of D-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 characteri ze SRIF antagonist activity. The hexapeptide library was screened in s tepwise iterative fashion to identify AC-178,335, a pure SRIF antagoni st of the sequence Ac-hfirwf-NH2. This D-hexapeptide bound SRIF recept or type 2 with an affinity constant (K-i) of 172 +/- 12 nM, blocked SR IF inhibition of adenylate cyclase in vitro (IC50 = 5.1 +/- 1.4 mu M), and induced GH release when given alone (50 mu g intravenously) to an esthetized rats with or without pretreatment with a long-acting SRIF a gonist.