ACTH production by the pars intermedia of the rainbow trout pituitary.

During in vitro culture, the pars intermedia of the rainbow trout pituitary synthesises and releases far greater amounts of immunologically and biologically active adrenocorticotrophin (ACTH) than does the pars distalis. This effect is inhibited by cycloheximide. Radio-immunossay of culture media for α-Melanocyte-stimulating hormone (α-MSH) and ACTH demonstrate the secretion also of trout α-MSH and a high molecular weight protein with ACTH immunoreactivity. The findings support the concept that ACTH and α-MSH may be derived from a common high molecular weight protein precursor. Some possible physiological implications of ACTH secretion by the pars intermedia are discussed.