“The clinical signs of acute laminitis, including a bounding digital pulse and increased hoof temperature, suggest a vascular component. It is postulated that there are blood flow abnormalities including increased capillary pressure, flow in arteriovenous anastomoses, and venoconstriction so lamellae are deprived of blood flow. It has also been suggested that fluid leaks from the capillaries into the restricted soft tissue space, which leads to edema and ischemia by compressing the small blood vessels. With carbohydrate-induced laminitis models, research has been conflicting with both increased and decreased hoof temperature before the onset of lameness, suggesting increased or decreased digital blood flow. However, increased hoof temperature with the onset of lameness suggests subsequent increased blood flow. Despite these discrepancies, there is little doubt that part of the inflammatory process within the lamellae during laminitis involves endothelial activation (Loftus et al., 2007a) and dysfunction (Eades etal., 2007).” Kentucky Equine Research
The endothelium is the layer of cells lining the interior of blood vessels. It is a very thin layer, only one cell thick, but has a critical role to play in both dilating and constricting blood vessels by sending a chemical message to the muscles in the walls of the vessels.
The endothelium controls both vasodilation – relaxes the vessels – and vasoconstriction – makes them more narrow. When insulin is working properly, it triggers the production of nitric oxide, a simple gas that is a very potent vasodilator. Endothelial nitric oxide also triggers healing and growth of new blood vessels.
The vasoconstricting substance released by the endothelium is called endothelin. Endothelin is the most potent vasoconstrictor known and structurally is very similar to things like snake venom and chemicals in bee stings. Studies have confirmed elevated levels of endothelin in horses with both acute and chronic laminitis.
When everything is working as it should, the endothelium reacts to a variety of triggers to keep the size of the blood vessels and therefore the blood flow through them at an appropriate level.
Dr. Eleanor Kellon, VMD