Final answer:
Sympathetic stimulation leads to the release of hormones that activate beta-adrenergic receptors, increasing cAMP levels. These increased levels of cAMP bind to HCN channels, enhancing their activity and ultimately contributing to a higher heart rate.
Step-by-step explanation:
Under sympathetic stimulation, the body releases hormones such as epinephrine and norepinephrine, which bind to beta-adrenergic receptors on cells. This binding action activates a G-protein, which in turn activates adenylyl cyclase, leading to an increase in the intracellular concentration of cAMP. cAMP acts as a second messenger and, among its roles, it positively modulates the activity of HCN channels. When cAMP levels rise, they bind to HCN channels and enhance their activity, leading to an increased heart rate.
Additionally, the amplification of the hormonal signal progresses as more and more G-proteins are activated by a single hormone-binding event, generating substantial amounts of cAMP. Protein kinases then catalyze further reactions once activated by cAMP. The breakdown of cAMP by phosphodiesterase (PDE) regulates and halts hormone activity to avoid overstimulation. Accordingly, the answer to the student's question is: a) cAMP increases, leading to enhanced HCN channel activity.