AJP - Endocrinology and Metabolism, Vol 254, Issue 6 E720-E725, Copyright © 1988 by American Physiological Society
Regulation of glycogen synthase in muscle and adipose tissue during fasting and refeeding
D. Antwi, J. H. Youn, N. S. Shargill, D. D. Lesikar and H. R. Kaslow
Department of Physiology and Biophysics, University of Southern California School of Medicine, Los Angeles 90033.
Using immunoblot analysis, we examined the electrophoretic mobility of
glycogen synthase from rat skeletal muscle and adipose tissue. Extracts
from muscle freeze clamped in situ contained at least three forms of
synthase with different electrophoretic mobilities. Extracts from adipose
tissue also contained multiple forms but lacked the form with greatest
mobility found in the muscle extracts. Phosphorylation at multiple sites of
glycogen synthase is known to deactivate the enzyme and retard its
electrophoretic mobility in sodium dodecyl sulfate gels. These results
suggest that there is very little or no dephosphorylated glycogen synthase
in adipose tissue and that phosphorylated forms of glycogen synthase
synthesize adipose tissue glycogen. Relative to control, it is known that
fasting decreases and refeeding increases glucose incorporation into
glycogen in rat epididymal adipose tissue but not skeletal muscle incubated
in vitro in the presence of insulin. Fasting did not change the
electrophoretic pattern of muscle synthase but decreased the relative
amount of adipose tissue forms with greater mobility. Refeeding increased
above control the relative amount of adipose tissue synthase with greater
mobility. These results indicate that changes in the phosphorylations that
retard mobility contribute to the effects of fasting and refeeding on
adipose tissue glycogen metabolism.
