Effect of Insulin (Fed State)
In the fed state, blood glucose concentration rises to 10–12 mmol/L. This serves as a powerful signal for insulin secretion by the pancreas. As a result, the insulin-to-glucagon ratio shifts in favor of insulin.
Insulin regulates metabolism through three key mechanisms:
- Transport level: Accelerates glucose uptake by insulin-dependent tissues (muscle and adipose tissue).
- Genetic level: Induces the synthesis of the enzyme glucokinase in the liver, accelerating glucose phosphorylation.
- Covalent modification: Activates phosphodiesterase (which decreases intracellular cAMP levels) and triggers the action of protein phosphatase-1 associated with glycogen particles (glycogen-associated protein phosphatase).
Insulin binding to its receptor initiates a cascade that converts glycogen phosphatase into an active form (at the expense of ATP). The active phosphatase removes inorganic phosphates from key enzymes. Consequently, glycogen synthase is dephosphorylated and becomes active, whereas glycogen phosphorylase is dephosphorylated and loses its activity. This initiates glycogen synthesis and halts its degradation.
Hormonal Regulation During Stress
Under acute stress, the blood concentration of epinephrine increases. The hormone binds to $\alpha_1$-receptors on hepatocyte membranes, activating the phosphoinositide signaling pathway.
This cascade involves the activation of phospholipase C, leading to two main effects:
- Mobilization of $Ca^{2+}$ ions from the endoplasmic reticulum.
- Activation of protein kinase C (PKC).
Active protein kinase C uses ATP to phosphorylate active glycogen synthase. This forms the phosphorylated, inactive form of the enzyme, completely blocking glycogen synthesis. Concurrently, epinephrine acts via the adenylate cyclase system: it promotes the phosphorylation of glycogen phosphorylase, converting it into its active form. The net result is a rapid cellular switch toward glycogen degradation.
Three Pathways for Glycogen Phosphorylase Activation
The initiation of glycogenolysis depends on the physiological state of the tissue (rest, exercise, stress). There are three main mechanisms for activating glycogen phosphorylase:
- Allosteric activation (during moderate exercise and rest). During muscle contraction, ATP hydrolysis leads to the accumulation of AMP. When cAMP levels are low, glycogen phosphorylase exists in a dephosphorylated (less active) form. AMP acts as an allosteric activator, initiating glycogenolysis without hormonal input.
- Calcium-dependent activation. Nerve impulses trigger the release of $Ca^{2+}$ ions from the sarcoplasmic reticulum. Calcium binds to the protein calmodulin. The resulting complex activates phosphorylase kinase, which in turn phosphorylates and activates glycogen phosphorylase.
- Epinephrine cascade. During intense muscle activity, epinephrine activates the same phosphorylase kinase via PKA. Pathways 2 and 3 frequently operate synergistically.
Changes in Hepatic Glycogen Metabolism
Carbohydrate metabolism in hepatocytes rapidly responds to physiological shifts. The table below outlines key changes in metabolic rates and substance concentrations.
| Physiological State | Blood and Hepatocyte Changes | Metabolic Rate |
|---|---|---|
| Fasting (Postabsorptive) | Glucagon $\uparrow$, Insulin $\downarrow$ | Synthesis $\downarrow$, Degradation $\uparrow$ |
| Fed (Absorptive) | Insulin $\uparrow$, Glucagon $\downarrow$, Glucose $\uparrow$<br>cAMP $\downarrow$, Glucose-6-phosphate $\uparrow$ | Synthesis $\uparrow$, Degradation $\downarrow$ |
| Stress | Epinephrine $\uparrow$<br>cAMP $\uparrow$, $Ca^{2+}$-calmodulin $\uparrow$ | Synthesis $\downarrow$, Degradation $\uparrow$ |