Final answer:
False. Epinephrine and norepinephrine do not act as agonists for all receptor sites; epinephrine activates alpha and beta receptors, while norepinephrine primarily targets alpha receptors.
Step-by-step explanation:
It is false that epinephrine and norepinephrine act as agonists for all receptor sites in which they bind. While both catecholamines can bind to adrenergic receptors and typically cause a sympathetic response, their selectivity varies. Epinephrine can activate both alpha and beta adrenergic receptors, whereas norepinephrine primarily targets alpha receptors. Beta-adrenergic receptors on cells are activated by these hormones, which then trigger a cascade of events in the cell, including the activation of a G-protein and adenylyl cyclase, leading to an increase in cAMP. This does not mean they are agonists for all receptor sites.