Cardiovascular markers during hyperinsulinemia in insulin resistant and insulin sensitive horses

Abstract: Background: Insulin is an important metabolic hormone that also has cardiovasculat effects. Insulin resistance is characterized by decreased tissue sensitivity to insulin, and is a part of two important endocrine diseases in horses, Equine Metabolic Syndrome and Pituitary Pars Intermedia Dysfunction. Both diseases carry an increased risk for laminitis. Decreased insulin sensitivity in people has been shown to lead to a decrease of the vasodilating effect of insulin. Increased baseline blood pressure and heart rate have also been seen in insulin resistant dogs and rats. The effect of induced hyperinsulinemia on cardiovascular markers such as heart rate, blood pressure and peripheral blood flow has been studied in healthy as well as insulin resistent people and laboratory animals. Studies regarding the effect of insulin resistance and induced hyperinsulinemia on cardiovascular markers in horses are scarce. The purpose of this study was to compare heart rate, blood pressure and hoof wall surface temperature (HWST) in insulin resistant horses with controls and to study the effect of hyperinsulinemia. Method: Ten insulin resistant (IR) horses and four healthy controls were included in the study. A 180 min euglycemic-hyperinsulinemic clamp (EHC) was performed in all horses. All horses were evaluated for body condition. Heart rate and systemic blood pressure was recorded before the start of the insulin infusion and at 45, 105 and 165 min of insulin infusion. Hoof wall surface temperature was recorded continuously during the 180 min EHC, starting 45 min before infusion start to obtain baseline temperatures. Blood samples for serum insulin and glucose were taken at 10 min intervals during the entire EHC, and insulin sensitivity was assessed from calculation of metabolized glucose per unit insulin (M/I ratio, with <5 considered insulin resistant). Results: Insulin resistant horses has higher baseline heart rates than control horses, and heart rate was positively associated with body condition score. There was no difference in systemic blood pressure or hoof wall surface temperatures between groups. Insulin infusion caused a decrease in systolic blood pressure in the control group, but not in the IR group. The induced hyperinsulinemia did not influence heart rate, mean arterial blood pressure, diastolic blood pressure or hoof wall surface temperature in either group. Conclusion: This study indicates that horses with insulin resistance have higher baseline heart rates compared to controls and that heart rate is associated with BCS. There are also indications that insulin resistant horses might have an altered cardiovascular response to hyperinsulinemia. Further studies are needed to validate this result, preferably with greater sample sizes and using different breeds of horses.