Chronic elevated calcium blocks AMPK-induced GLUT-4 expression in skeletal muscle

Area of Science:

• Exercise physiology
• Molecular biology
• Cellular metabolism

Background:

• Muscle contraction enhances glucose transport independently of insulin, crucial for energy homeostasis.
• Skeletal muscle glucose transporter type 4 (GLUT-4) expression is a key determinant of glucose uptake.
• Contraction-mediated signaling pathways involving calcium and AMP-activated protein kinase (AMPK) regulate GLUT-4.

Purpose of the Study:

• To investigate the impact of contraction-associated signals, calcium and AMPK, on glucose uptake and GLUT-4 expression.
• To differentiate the effects of acute versus chronic exposure to these signals.
• To elucidate the interplay between elevated cytosolic calcium and AMPK activation on GLUT-4 regulation.

Main Methods:

• Utilized C2C12 myotubes treated with caffeine (calcium-releasing agent) and 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR, AMPK activator).
• Employed dantrolene to investigate the role of ryanodine receptors in calcium's effects.
• Examined GLUT-4 mRNA and protein expression in genetically modified pigs with altered calcium and AMPK signaling.

Main Results:

• Low-concentration/short-term caffeine combined with AICAR showed additive effects on GLUT-4 expression.
• High-concentration/long-term caffeine treatment inhibited AICAR-induced GLUT-4 expression in a dose- and time-dependent manner.
• Elevated cytosolic calcium, particularly under chronic conditions, blunted AMPK-stimulated GLUT-4 expression in both cell culture and in vivo models.

Conclusions:

• Chronic elevation of cytosolic calcium negatively impacts AMPK-mediated GLUT-4 expression in skeletal muscle.
• This suggests a potential mechanism by which sustained high calcium levels could impair muscle glucose uptake.
• Understanding this interaction is vital for metabolic health and exercise physiology research.