AJP - Endocrinology and Metabolism, Vol 245, Issue 5 521-E527, Copyright © 1983 by American Physiological Society
Response of liver glycogen synthase and phosphorylase to in vivo glucose and glucose analogues
F. Q. Nuttall, J. W. Theen, C. Niewoehner and D. P. Gilboe
Glucose causes a rapid increase in the proportion (%) of glycogen synthase
in the active (I) form and a rapid decrease in the proportion of
phosphorylase in the active (a) form in both fed and fasted rats. The
changes in synthase I and phosphorylase a are more rapid in fasted animals.
With graded doses of glucose, the maximal decrease in phosphorylase a
occurred at a dose that was considerably smaller than that required to
maximally stimulate an increase in % synthase I. Thus, in the intact animal
a dissociation between the effects of glucose on the synthase and
phosphorylase systems was observed. Sorbitol, mannose, galactose, and
arabinose all stimulated an increase in synthase I but did not
significantly affect the proportion of phosphorylase in the a form. The %
synthase I was not significantly affected by a number of other glucose
homologues, pentoses, or three-carbon gluconeogenic substrates. The ketoses
fructose and mannoheptulose both caused a striking increase in %
phosphorylase a and a decrease in % synthase I, i.e., results opposite to
those of glucose. The mechanism by which fructose induces these changes is
not known, but the mannoheptulose effects may be accounted for by a rise in
liver cAMP concentration.
