Insulin stimulates GLUT4 translocation in the semitendinosus muscle of Shetland ponies.

The research article discusses a study that explored insulin-stimulated glucose uptake in the skeletal muscles of ponies by examining glucose transporter (GLUT4) expression and translocation. The findings show that while insulin does stimulate GLUT4 translocation in ponies, it is not as efficient as it is in other farm animals, potentially contributing to insulin resistance in ponies.

Introduction

• Insulin-regulated glucose uptake is a crucial factor in maintaining glucose homeostasis. It primarily occurs in the skeletal muscles and fat tissues through the translocation of the glucose transporter GLUT4 into cellular plasma membranes.
• This study aimed to better understand the expression and translocation of GLUT4, as well as GLUT1, and the uptake of glucose in the skeletal muscles of Shetland ponies.

Methodology

• Researchers took semitendinosus muscle explants from six Shetland pony geldings via open muscle biopsy under general anaesthesia.
• The expression of GLUT4 was analysed by measuring the GLUT4 protein contents in the muscle’s crude membrane (CM).
• The translocation of GLUT1 and GLUT4 in response to insulin was determined by measuring their concentrations in partially purified plasma membranes (PM) and cytoplasmic vesicles (CV), using a process referred to as semi-quantitative Western blotting.
• Insulin-stimulated glucose uptake was studied using a radioactive glucose analogue, 3-O-d-methyl[(3)H]glucose.

Results

• Incubating the muscle strips with 0.1 and 20mIU/mL insulin increased GLUT4 translocation as observed with the increase in PM GLUT4 content. However, it did not significantly impact GLUT4 expression or PM GLUT1 concentration.
• No increase emerged in the uptake of myocyte 3-O-Methylglucose following insulin stimulation.
• GLUT4 translocation in ponies is insulin-dependent, just like in rodents and farm animals. However, the efficiency of insulin-stimulated GLUT4 activation was noted to be lower in ponies than in pigs and cows. This lower efficiency could contribute to the insulin resistance observed in previous euglycaemic, hyperinsulinaemic clamp tests on Shetland ponies.

Conclusion

• The study’s results affirmed the appropriateness of the sub-cellular fractionation technique for determining insulin-stimulated GLUT4 translocation in equine skeletal muscle.
• Insulin-stimulated GLUT4 translocation in ponies is less efficient than in other farm animals, which contributes to a better understanding of insulin regulation in ponies and might explain insulin resistance observed in ponies.