Muscular Activity and the Cori Cycle:
Muscular activity or its anticipation leads to the release
of epinephrine by the adrenal medulla. Epinephrine markedly stimulates
in muscles and, to a lesser extent, in the liver. Muscular activity
quickly uses stored ATP as the energy source and more ATP must
be generated by the breakdown of glycogen. The sequence of epinephrine
stimulating events is outlined in the graphic on the left and
explained in the following sections.
The reaction cascade sequence is as follows:
1) Epinephrine binds to a receptor on the muscle cell membrane
and stimulates adenyl cyclase in the membrane.
2) Adenyl cyclase in the membrane catalyzes the formation
of cyclic AMP from ATP.
3) The increase of cyclic AMP activates a protein kinase.
The binding of cyclic AMP to an enzyme is an allosteric control
where the enzyme is "switched on" for activity.
4) The protein kinase causes phosphorylations (addition of
phosphate) on a series of phosphorylation enzymes which activates
them to finally produce glucose-1-phosphate.
4a) At the same time that enzymes are being activated for
glycogen breakdown, glycogen synthetase enzyme must be inactivated.
Glycogenesis must be "switched off" and glycogenolysis
5) Glucose-6-phosphate is the final result of the initial
stimulation by epinephrine or other hormones such as glucagon.
If this happened to a muscle cell, then the glycolysis pathway
is the next step in the sequence. If this happened to a liver
cell stimulated by glucagon, then glucose is produced to enter
the blood stream.