Which of the following effects of adrenaline would be blocked by phentolamine but NOT by propranolol ?
**Core Concept**
Adrenaline (epinephrine) is a catecholamine hormone and neurotransmitter 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 and propranolol are two different types of beta-blockers that selectively block different receptors.
**Why the Correct Answer is Right**
Phentolamine is a non-selective alpha-adrenergic receptor antagonist, meaning it blocks the action of adrenaline on alpha receptors. This results in the blockade of vasoconstriction, which is mediated by alpha-adrenergic receptors. On the other hand, propranolol is a beta-adrenergic receptor antagonist that selectively blocks the action of adrenaline on beta receptors. Therefore, phentolamine would block the vasoconstrictive effects of adrenaline, which are mediated by alpha-adrenergic receptors, but propranolol would not.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because propranolol would block the effects of adrenaline on beta receptors, which include vasodilation, increased heart rate, and increased contractility of the heart.
**Option B:** This option is incorrect because phentolamine would block the effects of adrenaline on alpha receptors, which include vasoconstriction, but propranolol would also block the effects of adrenaline on beta receptors.
**Option C:** This option is incorrect because propranolol would block the effects of adrenaline on beta receptors, which include increased heart rate and contractility of the heart.
**Option D:** This option is incorrect because phentolamine would block the effects of adrenaline on alpha receptors, which include vasoconstriction, but propranolol would not block the effects of adrenaline on alpha receptors.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that phentolamine is a non-selective alpha-adrenergic receptor antagonist, whereas propranolol is a selective beta-adrenergic receptor antagonist. This distinction is crucial in clinical practice, as phentolamine is used to treat conditions such as pheochromocytoma, whereas propranolol is used to treat conditions such as hypertension and angina.
**Correct Answer:** C.