Bioactive and immunoreactive adrenocorticotropin in normal equine pituitary and in pituitary tumors of horses with Cushing’s disease.

This research investigates the levels of bioactive and immunoreactive adrenocorticotropin (ACTH) in normal horse pituitary glands and horse pituitary tumors related to Cushing’s disease, revealing how the adenomas of horses with Cushing’s disease contain similar levels of bioactive ACTH as in normal glands, but higher volumes of inactive ACTH.

Understanding Equine Cushing’s Disease

• This study focuses on equine Cushing’s disease, a condition caused by an excessive secretion of adrenocorticotropin (ACTH), a hormone that helps regulate the body’s response to stress.
• This condition in horses occurs due to an overgrowth (hyperplasia) or tumors (adenomas) in the pars intermedia (PI) cells of the pituitary gland, contrasting with human Cushing’s disease which is triggered by similar hyperplasia or adenomas, but in the pars distalis (PD) cells.

Comparing ACTH Levels in Normal and Diseased Horses

• In this research, two types of ACTH were evaluated in normal equine pituitary glands and those from horses with Cushing’s disease – bioactive ACTH, which is metabolically active, and immunoreactive (IR) ACTH, which refers to ACTH that can trigger an immune response.
• The study found that in normal horse pituitary glands, most of the bioactive and immunoreactive ACTH was located in the PD, less in the PI and only trace amounts in the pars nervosa.
• However, in horses with Cushing’s disease, the concentration of bioactive ACTH in PI adenomas was similar to the levels found in normal PI tissues. What stood out though was that the total tumor content of bioactive ACTH exceeded that of a normal, whole pituitary gland.
• Results also showed that IR-ACTH levels were significantly higher in PI tumors, suggesting that a proportion of ACTH secreted by the tumor was biologically inactive.

Implications for Cushing’s Disease in Horses

• The research demonstrated that plasma IR-ACTH, which would presumably include a large fraction of bioactive ACTH, was greatly elevated in horses with Cushing’s disease.
• This excessive amount of active ACTH would explain the adrenal overgrowth (hyperplasia) and heightened function (hyperfunction) observed in affected horses. This, in turn, may lead to Cushing’s disease symptoms such as excessive thirst and urination, laminitis, muscle loss, and a pot-bellied appearance among other signs.