Which drug or intervention will cause the LEAST increase in heart rate in a transplanted denervated heart?

Which drug or intervention will cause the LEAST increase in heart rate in a transplanted denervated heart?

• A. Glucagon
• B. Atropine
• C. Isoproterenol
• D. Norepinephrine

Answer: (B)

Transplanted hearts are denervated, so their rate is not driven by autonomic nerve input but by circulating hormones and intrinsic pacemaker activity. Blocking parasympathetic muscarinic receptors with atropine removes any vagal braking, but since there is little or no vagal tone to begin with, atropine produces only a minimal change in rate. In contrast, drugs that act directly on the heart’s beta receptors or raise intracellular cAMP will push the rate up more noticeably: a direct beta-adrenergic agonist will cause a strong uptick in rate, norepinephrine increases rate via beta1 stimulation (despite vasoconstriction), and glucagon raises heart rate by increasing cAMP independently of adrenergic nerves. Therefore, atropine will cause the least increase in heart rate in a transplanted denervated heart.

Transplanted hearts are denervated, so their rate is not driven by autonomic nerve input but by circulating hormones and intrinsic pacemaker activity. Blocking parasympathetic muscarinic receptors with atropine removes any vagal braking, but since there is little or no vagal tone to begin with, atropine produces only a minimal change in rate. In contrast, drugs that act directly on the heart’s beta receptors or raise intracellular cAMP will push the rate up more noticeably: a direct beta-adrenergic agonist will cause a strong uptick in rate, norepinephrine increases rate via beta1 stimulation (despite vasoconstriction), and glucagon raises heart rate by increasing cAMP independently of adrenergic nerves. Therefore, atropine will cause the least increase in heart rate in a transplanted denervated heart.