**Core Concept**
The pacemaker potential in the sinoatrial (SA) node is generated by a slow depolarization phase, primarily due to the opening of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Vagal tone, mediated by the parasympathetic nervous system, influences the SA node's pacemaker potential through various ion channels and receptors.

**Why the Correct Answer is Right**
When vagal tone increases, it leads to the release of acetylcholine (ACh) in the SA node. ACh binds to muscarinic receptors (specifically, M2 receptors) on the SA node cells, which activates G-protein-coupled inward-rectifying potassium (GIRK) channels. The activation of GIRK channels increases the efflux of potassium ions (K+), resulting in hyperpolarization of the SA node cells. This hyperpolarization reduces the slope of phase 4 depolarization, thereby decreasing the heart rate. The increased vagal tone also reduces the expression of the pacemaker current (If), which is primarily carried by sodium (Na+) and calcium (Ca2+) ions. This reduction in If further contributes to the decrease in heart rate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because increased vagal tone does not lead to an increase in the pacemaker potential or an increase in the heart rate. Instead, it causes hyperpolarization and a decrease in the heart rate.

**Option B:** This option is incorrect because the pacemaker potential in the SA node is not primarily generated by the opening of L-type calcium channels. While calcium channels do play a role in the pacemaker potential, the slow depolarization phase is primarily due to the opening of HCN channels.

**Option C:** This option is incorrect because the effect of increased vagal tone on the pacemaker potential is not to increase the expression of the pacemaker current (If). Instead, it reduces the expression of If, contributing to the decrease in heart rate.

**Clinical Pearl / High-Yield Fact**
The SA node's pacemaker potential is influenced by various ion channels and receptors, including HCN channels, muscarinic receptors, and GIRK channels. Understanding the effects of vagal tone on these ion channels is crucial for appreciating the mechanisms underlying heart rate regulation.

**Correct Answer: A. Decrease in heart rate**