Met-enkephalin stimulates secretion of ACTH in sheep

The ability of opioid peptides to affect plasma immunoreactive ACTH concentrations was examined in conscious sheep. Plasma concentrations of ACTH were significantly increased by an i.v. infusion of Met-enkephalin and its analogue (FK-33824). There was a dose-dependent increase in plasma concentrations of ACTH with a graded administration of FK-33824. The combined effect of Met-enkephalin and ovine corticotrophin-releasing factor (oCRF) or FK-33824 and oCRF on plasma concentrations of ACTH was greater than the effect of each peptide when given individually. This study demonstrates that Met-enkephalin and its analogue stimulate ACTH release in sheep.