Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: Adrenaline should not be used to treat hypotension caused by phenothiazine derivatives.
Reason R: Phenothiazine derivatives cause alpha adrenergic blockade.
In light of the above statements, choose the correct answer from the options given below:

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

Remember the concept of "Adrenaline Reversal": always avoid adrenaline in animals treated with \(\alpha\)-blockers like acepromazine, and use phenylephrine or fluid therapy instead to manage low blood pressure.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Phenothiazine derivatives such as acepromazine are widely used as pre-anesthetic tranquilizers in veterinary medicine.
These agents act primarily as antagonists at peripheral post-synaptic \(\alpha_1\)-adrenergic receptors, leading to systematic vasodilation and hypotension.

Step 2: Detailed Explanation:
When phenothiazines block \(\alpha_1\)-adrenergic receptors, the normal vasoconstrictive mechanisms are temporarily disabled.
If adrenaline (epinephrine) is administered to correct this resulting hypotension, a physiological phenomenon known as "epinephrine reversal" occurs.
Adrenaline is a non-selective agonist acting on both \(\alpha\)-adrenergic receptors and \(\beta\)-adrenergic receptors.
Because the vasoconscriptive \(\alpha_1\) receptors are blocked by the phenothiazine, adrenaline can only bind to the unblocked vasodilatory \(\beta_2\)-receptors in skeletal muscle.
This selective stimulation of \(\beta_2\)-receptors causes profound systemic vasodilation, which further decreases total peripheral resistance.
As a direct consequence, the administration of adrenaline worsens the hypotension instead of correcting it.
To resolve phenothiazine-induced hypotension, pure \(\alpha_1\)-agonists like phenylephrine or noradrenaline should be utilized because they do not have significant \(\beta_2\) action.
Therefore, Assertion A is correct, Reason R is correct, and Reason R explains the physiological mechanism of Assertion A.

Step 3: Final Answer:
Both A and R are true and R is the correct explanation of A.