Final answer:
Beta-blockers, used to treat hypertension by blocking adrenaline's effect on the heart, can cause severe hypertension due to unopposed alpha-1 agonism in the case of pheochromocytoma and clonidine withdrawal. These drugs, such as metoprolol and propranolol, must be used cautiously to prevent adverse effects like bradycardia.
Step-by-step explanation:
Beta-blockers are a class of medications used to treat various cardiovascular conditions, including hypertension. They function primarily by inhibiting the effects of adrenaline on the heart by blocking beta-adrenergic receptors. These receptors include beta-1 receptors, which are primarily located in the heart and when stimulated by norepinephrine (NE), lead to an increase in heart rate and the strength of cardiac contractions. By blocking these receptors, beta-blockers can help to slow heart rate (HR) and lower blood pressure.
However, when beta-blockers are administered in the presence of conditions leading to high levels of circulating catecholamines like NE and adrenaline, they can cause unopposed alpha-1 adrenergic receptor activation. This can happen because beta-receptors are blocked, leaving the alpha receptors unopposed, which can lead to severe hypertension. The two scenarios where this may occur include pheochromocytoma, a tumor of the adrenal medulla that overproduces catecholamines, and during an event of withdrawal from clonidine, a medication that inhibits sympathetic outflow which can cause a rebound increase in catecholamines upon abrupt discontinuation.
Beta-blockers like metoprolol specifically block beta-1 receptors and are used to improve cardiac function. Another beta-blocker, propranolol, non-specifically blocks beta receptors. These drugs, when overprescribed, can lead to bradycardia or even cardiac arrest, demonstrating the importance of careful management of their administration.