Final answer:
Glucagon, epinephrine, and norepinephrine act as ligands for G-protein coupled receptors such as the glucagon receptor and beta-adrenergic receptors. These interactions initiate signaling pathways that regulate metabolism and the fight-or-flight response through the production of cAMP.
Step-by-step explanation:
Glucagon, epinephrine, and norepinephrine are ligands for specific G-protein coupled receptors (GPCRs). The glucagon receptor is activated by glucagon, which is a peptide hormone, and is predominantly expressed in the liver and kidney. This receptor stimulates adenylate cyclase when activated, increasing intracellular cAMP levels, which is critical for regulating metabolic processes such as glycogenolysis and gluconeogenesis.
Epinephrine and norepinephrine, on the other hand, bind to beta-adrenergic receptors as well as alpha-adrenergic receptors. These hormones are involved in the fight-or-flight response, modulating cardiovascular functions, energy metabolism, and other physiological processes through the activation of these receptors. Upon binding to the receptors, a cascade is triggered that also results in the production of cAMP, mediating various cell-specific responses.
Specifically, epinephrine binds to beta-adrenergic receptors on the plasma membrane, initiating a signaling cascade via G-proteins, which activate adenylyl cyclase, converting ATP to cAMP. This leads to changes in cellular activities, including an increase in glucose availability to the muscles and modulation of heart rate and force of contraction. Norepinephrine has similar effects but is more restricted in receptor binding, primarily interacting with alpha-adrenergic receptors.