Comparison of Insulin-Induced Digital Vessel Ring Responses of Laminitic and Clinically Healthy Horses

Hyperinsulinemia leads to insulin resistance of blood vessels and interferes with circulation of laminae of the equine foot. The objective of the study was to compare the digital vessel responses of clinically healthy and laminitic horses with and without experimental induction of insulin resistance. Vessel segments (artery and vein) collected after euthanasia were cut into 3-mm wide rings and were prepared in tissue baths containing Tyrode solution for response studies. Two rings of each vessel type from each horse were used as nonresistant, and two were made insulin resistant. Insulin resistance was induced by incubating the rings with 10-μM bath concentration of insulin. Then, all rings were contracted with phenylephrine (5 μM). When the contraction reached a plateau, insulin (10 μM) was added, and the response monitored for 30 minutes. The response to insulin was calculated as a percentage of the phenylephrine response. The results showed that the responses of the nonresistant rings differed significantly between the groups. However, when all rings were made insulin resistant, the significance between the groups disappeared. In the laminitic group, responses of resistant and nonresistant rings did not differ, whereas in the healthy group, the vessel responses differed significantly. In all experiments, arterial and venous rings followed the same pattern, but the magnitudes were greater in arterial rings. The findings suggest that the vessel responses to insulin are altered in laminitis which could be due to insulin resistance.