4. Which one of the following is NOT involved with the pacemaker potential of heart?
(1) “h”- channel
(2) Transient calcium channel
(3) Long-lasting calcium channel
(4) “f”-channel

The heart’s rhythmic beating relies on the pacemaker potential generated by specialized cells in the sinoatrial (SA) node. This pacemaker potential depends on the activity of several ion channels that regulate the flow of ions like sodium, potassium, and calcium across the cell membranes. Understanding which channels are involved in this process is crucial for appreciating how the heart maintains its automaticity.

Ion Channels Responsible for Pacemaker Potential

The pacemaker potential is a slow, spontaneous depolarization phase in pacemaker cells that leads to the generation of the heart’s rhythmic action potentials.

1. “h”-Channel (Hyperpolarization-Activated Channel)

• Also known as the HCN (hyperpolarization-activated cyclic nucleotide-gated) channel, the “h”-channel is crucial for initiating the pacemaker potential.

• It carries the “funny” current (I_f), which is a mixed sodium-potassium inward current activated at hyperpolarized membrane potentials (~ -60 mV).

• This channel helps bring the membrane potential towards the threshold for firing the next action potential, thus contributing directly to the pacemaker activity.pubmed.ncbi.nlm.nih+2

2. Transient Calcium Channel (T-type Calcium Channel)

• The transient or T-type calcium channels open briefly during the early phase of depolarization.

• They allow calcium ions (Ca²⁺) to enter the cell, further depolarizing the membrane towards the threshold.

• These channels contribute to the gradual depolarization phase of the pacemaker potential and help in reaching the action potential threshold.cvphysiology+1

3. Long-Lasting Calcium Channel (L-type Calcium Channel)

• The long-lasting or L-type calcium channels open at a more positive potential during late depolarization.

• These channels produce a sustained influx of calcium ions, responsible for the rapid upstroke (Phase 0) of the pacemaker action potential.

• They also contribute to excitation-contraction coupling in cardiac muscle cells.wikipedia+2

The “f”-Channel Is Not Separately Involved

• The “f”-channel often causes confusion with the “funny” current or “I_f”, which is actually carried by the HCN or “h”-channels.

• There is no separate “f”-channel distinct from the “h”-channel involved in pacemaker potential.

• Thus, the “f”-channel as a separate entity is not involved, making it the correct answer to the question of which is NOT involved.cvphysiology+1

Evaluating the Options

1. “h”-channel — Involved (carries funny current I_f)

2. Transient calcium channel — Involved (early depolarization)

3. Long-lasting calcium channel — Involved (late depolarization, action potential upstroke)

4. “f”-channel — Not involved as a separate channel; this term is sometimes a misnomer for the “h”-channel / funny current channel.

Overview of the Pacemaker Mechanism

The pacemaker cells at the SA node cycle through a repetitive process where:

• The h-channel generates an inward current that slowly raises membrane potential from the hyperpolarized state.

• T-type calcium channels and later L-type calcium channels facilitate further depolarization.

• Together, these changes bring the membrane potential to threshold, triggering an action potential that causes heart contraction.

• Potassium channels contribute to repolarization, resetting the cycle.

Summary

The heart’s pacemaker potential is generated by:

• “h”-channels (hyperpolarization-activated), which produce the funny current that initiates slow depolarization.

• Transient (T-type) calcium channels, which contribute to early depolarization.

• Long-lasting (L-type) calcium channels, which cause the rapid depolarization phase leading to action potential firing.

The “f”-channel is not a distinct channel involved in this process; it is a misnomer often confused with the “h”-channel.

Therefore, the correct answer for the channel NOT involved in the pacemaker potential is:

(4) “f”-channel.