Insulin and Laminitis
Insulin and Laminitis Scientists have taken another step towards unravelling the mystery of laminitis. Researchers in Australia have implicated insulin as a cause of laminitis. |  |
Normally, a meal containing sugar or starch leads to a rise in blood glucose, which stimulates the release of insulin, which in turn encourages the glucose into the cells. As a result, the blood glucose concentration returns to normal.
While many hormones, such as cortisol and adrenaline, act to raise the blood glucose concentration, only
insulin reduces them.
Many horses with laminitis are "insulin-resistant," especially those with underlying hormonal abnormalities ("endocrinopathic laminitis") such as equine metabolic syndrome (EMS) or Cushing's disease (ECS). They fail to respond normally to insulin, although rarely have a high blood glucose concentration.
Work at the University of Queensland has shown that insulin itself has a more important role in the development of laminitis than previously thought. Katie Asplin and her colleagues have found that prolonged high levels of the hormone can induce laminitis in ponies.
Ponies were given sufficient insulin by intravenous infusion to cause abnormally high blood levels.
Normally, this would be expected to cause a drop in the blood glucose concentration by stimulating the
uptake of glucose from the blood. But the scientists kept the blood glucose levels within the normal range by giving glucose as the same time.
These were young healthy ponies with no history of laminitis. All of them developed laminitis within three
days.
A second group of ponies received sterile saline instead of glucose and insulin. All had normal insulin and
glucose levels throughout the study. None of these ponies showed any sign of laminitis.
The only difference between the two groups was that the insulin-treated ponies had high levels of insulin in their blood. The average insulin concentration was 1036 U/mL, well above the normal upper limit.
The study showed that insulin could induce laminitis even if the blood glucose levels remained normal, and without any disturbance in hindgut function. It suggests that some physiological action of insulin could be a cause of equine laminitis
How could insulin damage the laminae? One suggestion is that insulin resistance might interfere with the
uptake of glucose by the lamellar epidermal basal cells. But recent work in the same laboratory has shown that these cells do not actually have insulin receptors. So how much glucose they take up does not depend on insulin.
However, there are insulin receptors in the blood vessels. So it is perhaps more likely that insulin exerts its effects by altering the blood flow to the foot.
Until now insulin has tended to be seen as an indicator of the metabolic changes present in horses with
laminitis. But it seems that it may itself be a significant cause of the damage that occurs in the disease.
Clearly insulin could play a major role in the development of endocrinopathic laminitis - those cases with insulin resistance and high blood levels of insulin. The researchers also suggest that it may also be important in the development of other types of laminitis. For example, that insulin may also be involved in many cases of pasture associated or dietary laminitis where rich pasture and or high concentrate feeding can induce insulin resistance in susceptible horses and push the insulin above a threshold level. They say that more work needs to be done to identify the threshold above which levels of insulin are toxic.
Horses with raised insulin levels should be managed to reduce the risk of laminitis developing - by switching from high carbohydrate to high fibre diets, aiming for progressive weight loss in overweight animals and giving appropriate exercise.
For more details see:
Induction of laminitis by prolonged hyperinsulinaemia in clinically normal ponies.
KE Aspin, MN Sillence, CC Pollitt, CM McGowan.
The Veterinary Journal (2007)
doi:10.1016/j.tvjl.2007.07.003
Reproduced with kind permission of Mark Andrews
Copyright: Mark Andrews - Equine Science
Added on: 15/02/08.
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