ACTH-releasing activity and cAMP accumulation of rat urocortin-related peptides in pituitary corticotropic cells

To investigate the structure-function relationship of urocortin (Ucn), ten Ucn-related peptides lacking C- or N-terminal amino acid residues were prep ared by solid phase technology. ACTH release and cAMP accumulation induced by these synthetic peptides were examined in the mouse anterior pituitary t umor cell line, AtT-20. Among ten Ucn-related peptides tested, Ucn(3-40) wa s found to be nearly equipotent to Ucn in the activities inducing ACTH rele ase and cAMP accumulation, suggesting that two N-terminal amino acids may n ot be crucial for these actions of Ucn. On the other hand, Ucn(23-40) and U cn(28-40) caused extremely low ACTH release. ACTH release was extremely low when AtT-20 was stimulated by synthetic peptides lacking six amino acids i n the C-terminal region of Ucn, such as Ucn(1-34), Ucn(3-34), Ucn(5-34), an d Ucn(8-34); and by synthetic peptides lacking C-terminal region of Ucn, Uc n(23-34) and Ucn(1-18). These results indicate that the C-terminal hexapept ide region of Ucn is of extreme importance for the activity of Ucn to cause ACTH release. Little, if any, cAMP accumulation was observed in the presen ce of Ucn(5-34), Ucn(8-34), Ucn(23-34) or Ucn(1-18), supporting clearly tha t the C-terminal 35-40 sequence of Ucn may be responsible for CRF/Ucn recep tor binding.