Amino Acid Profiles in PPID Horses: Management, Diagnostic Implications

Significant differences in amino acid concentrations of horses with pituitary pars intermedia dysfunction (PPID, Cushing’s disease) were recently identified.^* This finding may lead to novel diagnostic and treatment options, as well as an improved understanding of how PPID develops and progresses.

 This new knowledge dovetails with what is known about Parkinson’s disease in humans. One human study found that the amino acids arginine, alanine, and phenylalanine were significantly lower with advanced disease, suggesting these amino acid changes could serve as early markers of disease progression. With this in mind, German researchers theorized that different amino acid profiles may exist for healthy and PPID-affected horses. Treatment with pergolide may also alter amino acid concentrations, and those altered amino acid profiles could potentially be used as diagnostic markers of disease.

To compare amino acid profiles among healthy and PPID horses, blood samples from 53 apparently normal horses were collected. Adrenocorticotropic hormone (ACTH) blood levels were used to diagnose PPID. If the horse was already being treated for PPID with pergolide, then ACTH levels were used to determine if the condition was well regulated or not.

In total, 20 horses were diagnosed with PPID based on an ACTH concentration of 100 pg/mL or higher, and 25 were diagnosed as non-PPID with ACTH concentrations of 30 pg/mL or lower. For horses previously diagnosed and treated with pergolide, five had normal ACTH levels and were well regulated and three had elevated ACTH levels and were poorly regulated.

The most important differences in amino acid concentrations were:

• Arginine and citrulline were both significantly higher in well-regulated PPID horses than horses with and without PPID not on pergolide;
• Asparagine was significantly lower in horses without PPID than the other three groups;
• Cysteine was lower in the well-regulated PPID horses than the other three groups; and
• The excitatory amino acid glutamine was significantly higher in untreated PPID horses and PPID horses on pergolide that were poorly controlled.

“Of those five amino acids, arginine, citrulline, and glutamine may serve as diagnostic markers. Further, monitoring amino acid profiles may also be used as markers of disease severity,” said Kathleen Crandell, Ph.D., a Kentucky Equine Research nutritionist.

Glutamine is believed to be involved in regulating muscle synthesis and degradation, and muscle atrophy and weight loss are common clinical signs in horses with PPID. Therefore, finding altered glutamine concentrations between the different groups of horses was not unexpected.

“This study supports the hypothesis that amino acid profiles do differ between non-PPID and treated/untreated PPID horses. Additional research may reveal novel amino acid supplement strategies that could benefit horses with PPID,” Crandell suggested.

^*Stoeckle, S.D., D. Timmermann, R. Merle, and H. Gehlen. 2022. Plasma amino acids in horses suffering from pituitary pars intermedia dysfunction. Animals (Basel) 12(23):3315.