Which of the following effects of adrenaline would be blocked by phentolamine but NOT by propanolol?
**Core Concept**
Adrenaline (also known as epinephrine) is a catecholamine hormone that plays a crucial role in the body's fight-or-flight response. It acts on various receptors, including alpha-adrenergic receptors, to produce its effects. Phentolamine is a non-selective alpha-adrenergic receptor antagonist, while propranolol is a non-selective beta-adrenergic receptor antagonist.
**Why the Correct Answer is Right**
Phentolamine works by competitively inhibiting the binding of adrenaline to alpha-adrenergic receptors, thereby blocking its effects on these receptors. Adrenaline's effects on alpha-adrenergic receptors include vasoconstriction, which leads to increased blood pressure. Propranolol, on the other hand, blocks the effects of adrenaline on beta-adrenergic receptors, which include tachycardia and vasodilation. However, propranolol does not block the effects of adrenaline on alpha-adrenergic receptors.
**Why Each Wrong Option is Incorrect**
* **Option A:** Blocking the effects of adrenaline on beta-adrenergic receptors, which is the action of propranolol, not phentolamine.
* **Option B:** Blocking the effects of adrenaline on alpha-adrenergic receptors is the action of phentolamine, which is correct, but the question asks what phentolamine would block that propranolol would not.
* **Option D:** This option is incomplete.
**Clinical Pearl / High-Yield Fact**
Phentolamine is often used to treat hypertension caused by pheochromocytoma, a type of tumor that secretes excess adrenaline. It is also used in anesthesia to reverse the effects of adrenaline on alpha-adrenergic receptors, which can cause vasoconstriction and increase blood pressure.
**Correct Answer:** C.