Hyperinsulinaemia increases vascular resistance and endothelin-1 expression in the equine digit

Reasons for performing study: Insulin leads to overexpression of endothelin-1 (ET-1) in the endothelium of insulin-resistant rodents. If this is also the case in equine laminar tissue, this could explain the predisposition of insulin-resistant horses to laminitis. Objectives: To investigate the effect of hyperinsulinaemia on metabolism and vascular resistance of the isolated equine digit in a model of extracorporeal perfusion. Study design: Randomised, controlled study with interventional group, with blinded evaluation of histology results. Method: After exsanguination, equine digits (n = 11) and autologous blood were collected at an abattoir. One digit served as a hyperinsulinaemic pilot limb, 5 digits were assigned to the hyperinsulinaemic perfusion (IP) group and 5 to the control perfusion (CP) group. Digits were perfused for 10 h at a defined perfusion rate of 12 ml/min/kg. After the first hour of perfusion (equilibration period), insulin was added to the reservoir of the IP digits. Perfusion pressure, glucose consumption, lactate and lactate dehydrogenase were monitored. Vascular resistance was calculated as perfusion pressure (in millimetres of mercury) in relation to the flow rate (in millilitres per minute). After perfusion, histology samples of the dorsal hoof wall (haematoxylin & eosin or periodic acid-Schiff) were evaluated. Immunohistology with a polyclonal rabbit-derived anti-endothelin antibody was used for detection of ET-1. Results: In the IP group, the mean insulin concentration in the plasma of the perfusate was 142 +/- 81 mu iu/ml, while insulin concentration was <3 miu/ml in the CP group. Mean vascular resistance was significantly higher (P<0.01) in the IP group (2.04 +/- 1.13 mmHg/ml/min) than in the CP group (1.31 +/- 0.55 mmHg/ml/min). Histology of the IP group samples showed significantly more vessels with an open lumen, increased width of the secondary epidermal lamellae and formation of oedema. In the lamellar vessels (veins and arteries) and nerve fibres, ET-1 expression was much more prominent in the IP group than in the CP group samples. Conclusions: Short-termhyperinsulinaemia leads to increased vascular resistance in the equine digit and increased expression of ET-1 in the laminar tissue.