Which statement correctly describes the receptor pharmacology responsible for the haemodynamic profile of dobutamine in low-output states?
- A Pure beta-1 agonism with no activity at other adrenoceptors
- B A racemic mixture where opposing alpha-1 actions offset each other, leaving predominant beta-1 stimulation ✓
- C Selective beta-2 agonism producing vasodilatation and increased cardiac output
- D Mixed alpha-1 and alpha-2 agonism with weak beta activity
Correct answer: B. A racemic mixture where opposing alpha-1 actions offset each other, leaving predominant beta-1 stimulation
Explanation
Dobutamine is a racemic mixture. The (+) enantiomer is a potent beta agonist and an alpha-1 antagonist, while the (-) enantiomer is an alpha-1 agonist. These opposing alpha-1 actions largely cancel out, so the net clinically relevant effect is strong beta-1 stimulation producing increased contractility and cardiac output with relatively modest tachycardia compared with isoproterenol. Describing it as pure beta-1 agonism ignores this important stereochemical basis.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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