Effects of acute dopamine infusion on metabolic and endocrine responses in horses with different endocrine phenotypes: an exploratory fixed-sequence study.

Overview

• This study investigated how acute dopamine infusion affects insulin and metabolic responses in horses with differing levels of insulin sensitivity and insulin secretion.
• The research aimed to clarify dopamine’s effects on horse metabolism by comparing metabolic markers during dopamine infusion versus control conditions in the same animals.

Background

• Dopamine is known to influence insulin secretion and sensitivity in species like humans and rodents, but its metabolic impact in horses is not well-understood.
• Horses exhibit varying endocrine phenotypes related to insulin dynamics, such as normoinsulinemia (normal insulin levels) or hyperinsulinemia (elevated insulin levels), as well as insulin sensitivity or resistance.
• Understanding dopamine’s role could provide insights into managing equine metabolic disorders like insulin resistance, which relates to equine metabolic syndrome.

Study Design and Methods

• Seven horses were studied, each acting as their own control to reduce variability.
• Two main metabolic tests were performed on each horse:
  • Oral Sugar Test (OST): Assesses insulin secretion in response to carbohydrate ingestion.
  • Hyperinsulinemic-Euglycemic Clamp (EHC): Measures insulin sensitivity by maintaining constant high insulin and glucose levels and monitoring glucose infusion rates (GIR).
• Each test was conducted twice per horse:
  • Under normal conditions (control).
  • During continuous infusion of dopamine hydrochloride (DA HCl) at 2.5 μg/kg/min.
• Horses were separated into groups based on:
  • OST results: normoinsulinemic (NI, n=4) vs. hyperinsulinemic (HI, n=3).
  • EHC results: insulin-sensitive (IS, n=3) vs. insulin-resistant (IR, n=4), classified based on glucose infusion rates during control EHC.

Key Findings

• During the oral sugar test (OST):
  • Dopamine infusion did not significantly change blood glucose or insulin levels.
  • Areas under the curve (AUC) for both glucose and insulin were similar between dopamine and control conditions.
• During the hyperinsulinemic-euglycemic clamp (EHC):
  • Exploratory analysis revealed that dopamine’s effect on glucose infusion rate (GIR) and an insulin sensitivity index (M/I) was dependent on the horse’s insulin sensitivity classification.
  • However, no statistically significant effects were observed within individual groups, possibly due to small sample sizes.
• Dopamine infusion markedly reduced circulating prolactin levels, confirming the biological activity of the dopamine treatment.

Interpretation and Implications

• The lack of effect on insulin secretion during OST suggests dopamine does not acutely modulate the horse’s insulin response to oral carbohydrates.
• The differences noted in insulin sensitivity measures during EHC hint that dopamine’s impact might vary depending on the horse’s underlying insulin sensitivity status.
• Marked prolactin suppression confirms the dopamine infusion was effective systemically, validating the physiological relevance of the treatment.
• The study’s exploratory nature, small sample size, and fixed treatment order limit the robustness of conclusions.
• Results highlight the potential complexity of dopamine’s metabolic effects in horses and the need for larger, randomized controlled studies to confirm these preliminary observations.

Limitations

• Small number of horses (n=7) reducing statistical power.
• Fixed treatment sequence (control always before dopamine), which can introduce order effects.
• Classification into insulin-sensitive or resistant was based on study-specific cutoffs, potentially limiting generalizability.
• Multiple comparisons increase the risk of type I errors (false positives).

Conclusions

• Acute peripheral dopamine infusion reduces prolactin in horses but does not alter insulin secretion in response to oral carbohydrates.
• Dopamine’s effects on peripheral insulin sensitivity may differ according to the horse’s metabolic phenotype, warranting further study.
• More extensive randomized trials are needed to confirm and extend these findings and to understand the therapeutic implications for equine metabolic health.