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Biochemical Titration of Glycogen In vitro
Published on: November 24, 2013
Cell volume affects glycogen phosphorylase activity in fish hepatocytes
N K Hallgren1, E R Busby, T P Mommsen
1Department of Biochemistry and Microbiology, University of Victoria, V8W 3P6, Victoria, BC, Canada.
Summary
Cell hydration affects glycogen phosphorylase a (GPase a) activity. Hyperosmotic stress activates GPase a, increasing glucose production, while hyposmotic stress decreases activity.
Area of Science:
- Biochemistry
- Cell Physiology
- Comparative Physiology
Background:
- Glycogen phosphorylase (GPase) activity is crucial for glucose metabolism.
- Hepatocyte hydration state is known to influence GPase activity.
Purpose of the Study:
- To investigate the relationship between hepatocyte osmolarity and GPase a activity.
- To explore the mechanisms and implications of GPase a activation by osmotic stress.
Main Methods:
- Isolated hepatocytes from catfish (Ameiurus nebulosus) were exposed to varying medium osmolarities.
- GPase a activity, phosphorylation status, and cell volume were measured.
- Comparisons were made with glucagon stimulation and other fish species.
Main Results:
- GPase a activity showed a linear correlation with osmolarity in hyperosmotic conditions (254–478 mosmol l⁻¹), increasing up to 7-fold.
- Cell shrinkage preceded GPase a activation, which peaked within 10 minutes.
- Hypoosmotic conditions led to decreased GPase a activity and glucose output, with recovery lagging behind cell volume regulation.
Conclusions:
- Hepatocyte GPase a activity is directly regulated by extracellular osmolarity, independent of major signaling pathways like PKA or Ca²⁺.
- Osmotic stress-induced GPase a activation enhances glucose production, potentially supporting extrahepatic metabolic demands.
- This osmotic regulation of GPase phosphorylation is conserved across different fish species.
