Ion Channel Receptors

Have you ever wondered how we perceive a sour or salty taste? Our taste buds contain cell-surface receptors that respond to taste stimuli. Sour and salty taste stimuli are detected by cell-surface receptors called ion-channel-linked receptors. These are receptors that are bound by ligands that elicit the opening of an ion channel, which can then let in (or out!) ions like sodium, calcium, potassium, and more!

In this article, we will first define ion channels and cell signaling. Then we will go through the two types of receptors and identify what type of receptor ion-channel-linked receptors fall under. We will also discuss how these receptors are activated. Finally, we will cite some examples of ion-channel-linked receptors.

Ion channel receptors: Iron channel definition biology

Ion channels are channel proteins embedded in the plasma membrane. These ion channels facilitate the movement of ions across the membrane by acting as gated channels that open and close in response to certain stimuli. These stimuli are typically chemical or electrical in nature.

Ion channel receptors: What is cell signaling?

Cell signaling is the process by which a cell responds to messages from its external environment through protein receptors. The process of cell signaling is initiated when a signaling molecule called ligand binds to a protein receptor found inside or on the surface of the target cell. Cell signaling results in the initiation of a specific cellular function such as cell division and cell death.

There are two general types of cell receptors: internal and cell-surface receptors.

• Internal receptors (also known as cytoplasmic or intracellular receptors) can be found in the cytoplasm. Internal receptors bind hydrophobic (or non-polar) ligands such as steroid hormones. Unlike cell-surface receptors, internal receptors can travel across the plasma membrane.

• Cell-surface receptors–including ion-channel-linked receptors–span the plasma membrane, such that each receptor has extracellular, transmembrane, and cytoplasmic or intracellular regions. Unlike internal receptors, ligand-bound cell-surface receptors are not required to enter the cell. Instead, their extracellular signals need to be converted into intracellular signals in a process called signal transduction.

What is the definition of ion channel linked receptors in biology?

Ion-channel-linked receptors are a type of cell-surface receptor that, upon binding a ligand, change shape such that a channel is formed across the plasma membrane, allowing specific ions to pass through (Fig. 1).

Figure 1. Once a ligand binds to an ion linked channel receptor, the channel proteins change shape, allowing ions to pass through.

Ion-channel-linked receptors have a large membrane-spanning region which enables them to create a channel. Most of the amino acids in the membrane-spanning region are hydrophobic, so they are able to interact with the phospholipid fatty acid tails that make up the center of the plasma membrane. On the other hand, the amino acids that line the interior of the channel tend to be hydrophilic and polar, so water or ions are able to pass through.

Ion channel gating can be further subdivided into two general categories based on the initiating stimulus: ligand-binding and membrane voltage.

What is the mechanism behind the opening and closing of ligand-gated and voltage-gated ion channel receptors?

When a ligand binds to the extracellular area of the ligand-gated ion channel receptor, the channel protein uses mechanical work to reconfigure its structure, allowing it to switch between an open or closed state, thereby allowing or blocking the movement of ions like Na+ or Ca2+.

In contrast to ligand-gated channels, voltage-gated channels (also known as voltage-dependent ion channels) open or close in response to membrane voltage. Voltage-gated ion channels have an integral membrane ‘voltage sensor’ region in each of its subunits. A voltage sensor has the ability to convert energy stored in the electric field of the membrane into mechanical energy to change between open and closed states.

The distinction between ligand-gated and voltage-gated ion channels boils down to the type of interaction between the gating and the pore domains: whether electromechanical (voltage-gated) or chemomechanical (ligand-gated).

Ion channel linked receptors example 1: ligand gated and voltage gated sodium and calcium ion channels

Sodium (Na+) and calcium (Ca2+) ion channels are members of a family of ion channels called tetrameric cation channels.

Tetrameric cation channels have four membrane-spanning regions that surround a central channel that is selective for certain cations. Cation selectivity is thought to be an intrinsic property of the structure of the "pore". This selectivity is caused by charged amino acids lining the channel pore, which can then act as the voltage sensor for membrane voltage changes. Because gating domains are attached to the pore, tetrameric cation channels are conferred with the ability to “gate.”

In nerve, muscle, and other electrically excitable cells, voltage-gated sodium channels open when the membrane potential in their surroundings become depolarized, allowing the flow of positive sodium ions into the cell, in the direction of the electrochemical gradient (that is, from the region of higher to lower solute concentration and energy). The electrical signal transmitted through this process can result in an action potential that subsequently transforms into a cellular response such as the release of neurotransmitters. In a matter of milliseconds, these sodium channels inactivate, restoring sodium conductance to near-baseline levels.

Voltage-gated sodium channels have two gates: an activating gate that is voltage-dependent and an inactivating gate that is time-dependent.

• When the activating gate opens, sodium ions are able to enter and depolarize the cell. Regardless of the state of the activation gate, when the inactivation gate closes, the influx of sodium ions stops.

• These two gates work together to ensure that depolarization is controlled: after a few milliseconds, the voltage-gated sodium channels inactivate, stopping the flow of sodium even when stimulation persists. The channel will remain closed until the cell repolarizes to a threshold voltage that varies according to cell type.

Likewise, many types of excitable cells activate voltage-gated calcium channels, which transport calcium into cells, initiating a wide range of physiological functions such as contraction, neurotransmission, secretion, and gene transcription. Calcium currents were first detected in the heart muscle, where depolarization tends to activate voltage-gated calcium-selective channels. This explains why conventional calcium channel blockers are typically used to treat cardiovascular problems such as cardiac arrhythmia and hypertension.

Ion channel linked receptors example 2: receptors and sense of taste in humans

Receptor cells in mammals that are responsible for the sense of taste are basically modified epithelial cells that are organized into taste buds scattered all over the tongue and mouth. All five taste types (eg, bitter, salty, sweet, sour, umami), it turns out, have corresponding cell-surface receptors. Each receptor responds to a specific stimulus called a tastant. The five tastes are transmitted through various mechanisms that reflect the molecular composition of the tastant.

Sweet, umami, and bitter tastes each require one or more genes encoding for G protein-coupled receptors (GPCR).

On the other hand, sour taste ion channel receptors belong to the thermo-receptor proteins (TRP) family. The TRP proteins of the sour receptor form an ion channel in the plasma membrane of the taste cell. When an acid or other sour-tasting substance binds to the receptor, it causes the ion channel to change shape and these increase hydrogen ion (H+) concentrations in the taste cells, which depolarizes and excites them.

And as we might expect, the receptor for salty tastants turns out to be a sodium channel which detects sodium salts such as sodium chloride (also known as table salt).