After exercise, low levels of blood glucose can result in the release of glucagon (a hydrophilic hormone) from the pancreas - VCE - SSCE Biology - Question 3 - 2014 - Paper 1

Hydrophilic hormones like glucagon stimulate metabolic processes by binding to specific receptors on the target cell's surface. This interaction triggers a signal transduction pathway involving second messengers such as cyclic AMP (cAMP).

1. Binding to Receptors: Glucagon, upon release from the pancreas, circulates in the bloodstream and binds to specific glucagon receptors on liver cells. This is crucial as the receptors are specific to the hormone, ensuring the correct physiological response.

2. Activation of cAMP: The binding of glucagon activates an enzyme called adenylate cyclase, which converts ATP to cAMP. This increase in cAMP levels serves as a secondary messenger, amplifying the original signal.

3. Activation of Enzymes: cAMP activates protein kinase A (PKA), which in turn phosphorylates other enzymes involved in glycogenolysis. This includes the activation of glycogen phosphorylase, which facilitates the breakdown of glycogen into glucose.

4. Release of Glucose: The metabolic pathway leads to the release of glucose into the bloodstream, thus increasing blood glucose levels. This is a critical response in managing energy needs post-exercise.