Distribution of insulin receptor and insulin-like growth factor-1 receptor in the digital laminae of mixed-breed ponies: An immunohistochemical study

Reasons for performing study: Hyperinsulinaemia has been implicated in the pathogenesis of laminitis; however, laminar cell types responding to insulin remain poorly characterised. Objectives: To identify laminar cell types expressing insulin receptor (IRc) and/or insulin-like growth factor-1 receptor (IGF-1R); and to evaluate the effect of dietary nonstructural carbohydrate (NSC) on their expression. Methods: Mixed-breed ponies (n = 22) received a conditioning hay chop diet (NSC approximate to 6%); following acclimation, ponies were stratified into lean (n = 11, body condition score [BCS]4) or obese (n = 11, BCS 7) groups and each group further stratified to remain on the low NSC diet (n = 5 each for obese and lean) or receive a high NSC diet (total diet approximate to 42% NSC; n = 6 each for obese and lean) for 7 days. Laminar samples were collected at the end of the feeding protocol and stained immunohistochemically for IRc and IGF-1R. The number of IRc(+) cells was quantified; distribution of IGF-1R was qualitatively described. Laminar IRc content was assessed via immunoblotting. Results: The number of IRc(+) cells was greater in the laminae of high NSC ponies than low NSC ponies (P = 0.001); there was a positive correlation between the change in serum insulin concentration and number of IRc(+) cells (r2= 0.74; P<0.0001). No epithelial IRc(+) cells were observed; IRc(+) cells were absent from the deep dermis. Analysis of serial sections identified IRc(+) cells as endothelial cells. The distribution of IGF-1R was more extensive than that of IRc, with signal in vascular elements, epithelial cells and fibroblasts. Conclusions: Increased dietary NSC results in increased laminar endothelial IRc expression. Laminar keratinocytes do not express IRc, suggesting that insulin signalling in laminar epithelial cells must be mediated through other receptors (such as IGF-1R). Potential relevance: Manipulation of signalling downstream of IRc and IGF-1R may aid in treatment and prevention of laminitis associated with hyperinsulinaemia.