Characterization of insulin and IGF-1 receptor binding in equine liver and lamellar tissue: implications for endocrinopathic laminitis.

This research article explores how insulin and Insulin-like growth factor 1 (IGF-1) influence a common hoof affliction in horses, known as laminitis. The study found that while there was an abundance of IGF-1 receptors in the affected hoof tissue, insulin’s binding to these receptors was unlikely to be a main factor causing laminitis.

Introduction and Background

• The hoof disease, equine laminitis, is known to be triggered by hyperinsulinemia – a condition characterized by excessively high levels of insulin in the blood – but the exact mechanism remains unclear.
• Previous studies have shown that high concentrations of insulin lead to the separation of the weight-bearing structures from the hoof wall, leading to laminitis. There is also an observed increase in cell death and proliferation within the lamellar tissue of the hoof.
• Prior gene expression and immunohistochemistry studies have indicated that insulin receptors are not abundant in lamellae, while IGF-1 receptors are plentiful.

Main Research Findings

• In order to investigate whether insulin could be acting through IGF-1 receptors, the researchers utilized cell membrane fragments from the livers of 6 horses, and lamellae from over 50 horses which were euthanized for nonresearch reasons. These fragments contained IGF-1 receptors.
• Through radioligand-binding studies, the team confirmed the high presence of IGF-1 receptors in both the liver and lamellae (measured in femtomoles [fmol] per milligram of protein).
• However, the affinity of insulin for binding to the lamellar IGF-1 receptors was measured and found to be significantly lower than that of IGF-1. In fact, it was over 5,800 times lower, indicating that at physiological concentrations, insulin is unlikely to bind to IGF-1 receptors.

Additional Research Findings

• Insulin receptors, while detectable in the liver, were barely discernible in lamellae. This also contributes to the argument that insulin is unlikely to have a significant effect on lamellar IGF-1 receptors.
• The research did not find any evidence supporting the presence of hybrid receptors – ones that could bind both insulin and IGF-1 – within the lamellae or liver tissue.

Conclusions

• Contrary to the initial hypothesis, the study results suggest that insulin does not interact significantly with IGF-1 receptors, indicating that they are not a main factor causing laminitis.
• This conclusion reinforces the need for alternative theories to understand how insulin triggers this harmful hoof condition in horses.