Dobutamine is administered clinically as a racemic mixture. The pharmacodynamic basis for its net beta-1 agonist effect is:

• A Both enantiomers are equally potent beta-1 agonists
• B The dextro enantiomer inhibits norepinephrine reuptake while the levo enantiomer stimulates beta-1 receptors
• C The levo enantiomer is a potent beta-1 agonist while the dextro enantiomer is an alpha-1 antagonist ✓
• D Both enantiomers act exclusively through dopamine D1 receptors

Explanation

Dobutamine is a racemic mixture in which the (-) enantiomer is a strong beta-1 and alpha-1 agonist while the (+) enantiomer is a weak beta-1/beta-2 agonist and a potent alpha-1 antagonist. The opposing alpha effects largely cancel, leaving net beta-1 mediated positive inotropy with minimal peripheral vasoconstriction. Neither enantiomer acts through dopamine receptors, and the mixture is not simply additive beta-1 stimulation.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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