The purpose of this study was to determine the effects of prolonged administration of insulin, whilst maintaining normal glucose concentrations, on hoof lamellar integrity in vivo on healthy ponies with no known history of laminitis or insulin resistance. Nine clinically healthy, unrelated ponies were randomly allocated to either a treatment group (n = 5; 5.9 ± 1.7 years) or control group (n = 4; 7.0 ± 2.8 years). The treatment group received insulin via a euglycaemic hyperinsulinaemic clamp technique modified and prolonged for up to 72 h. Control ponies were infused with an equivalent volume of 0.9 % saline. Ponies were euthanased at the Obel grade 2 stage of clinical laminitis and hoof lamellar tissues were harvested and examined for histopathological evidence of laminitis.
Basal serum insulin and blood glucose concentrations were 15.7 ± 1.8 μU/mL and 5.2 ± 0.1 mmol/L, respectively (mean ± SE) and were not significantly different between groups. Mean serum insulin concentration in treatment ponies was 1036 ± 55 μU/mL vs. 14.6 μU/mL in controls. All ponies in the treatment group developed clinical and histological laminitis (Obel grade 2) in all four feet within 72 h (55.4 ± 5.5 h), whereas none of the control ponies developed laminitis. There was no clinical evidence of gastrointestinal involvement and the ponies showed no signs of systemic illness throughout the experiment. The data show that laminitis can be induced in healthy young ponies, with no prior history of laminitis, by maintaining prolonged hyperinsulinaemia with euglycaemia. This suggests a role for insulin in the pathogenesis of laminitis, independent of hyperglycaemia, or alterations in hind-gut fermentation. For the clinician, early detection and control of hyperinsulinaemia may facilitate management of endocrinopathic laminitis.
