Membrane reception
Membrane receptors are located on the cell surface and bind to protein-peptide hormones, catecholamines, and neurotransmitters. These ligands are hydrophilic and cannot pass through the lipid bilayer of the membrane on their own.
There are two main groups:
- Ionotropic receptors. These are proteins that function simultaneously as both a receptor and an ion canal. When a ligand binds to them, the channel opens immediately, generating an ion current (e.g., nicotinic acetylcholine receptors).
- Metabotropic receptors. Their activation triggers a cascade of intracellular secondary messengers (mediators).
Metabotropic receptors, in turn, are divided into catalytic (containing or linked to an enzyme) and G protein-coupled receptors (GPCRs). The second group is the most numerous; these receptors span the membrane 7 times and utilize G proteins to transmit signals into the cell.
Second messenger systems
In membrane reception, the signal is transmitted via a cascade of reactions. The G protein acts as a link between the receptor and intracellular systems.
One of the most important pathways is the adenylyl cyclase pathway:
- The G protein acts on the enzyme adenylyl cyclase.
- Adenylyl cyclase converts ATP into the secondary messenger — cAMP.
- cAMP activates cAMP-dependent protein kinases.
- Protein kinases phosphorylate proteins (usually enzymes), altering their activity and eliciting a cellular response.
The primary property of such cascades is the massive amplification of the primary signal, which allows hormones to function even at very low concentrations in the blood.
Intracellular reception
This mechanism is characteristic of lipophilic steroid and thyroid hormones. They are capable of penetrating inside the cell.
The process is as follows:
- The hormone dissociates from blood transport proteins and crosses the cell membrane.
- In the cytoplasm (or nucleus), it binds to its intracellular receptor.
- The hormone-receptor complex binds to DNA.
- Transcription is activated (mRNA synthesis), followed by translation — the synthesis of new proteins on ribosomes.
- The new proteins alter cell function.
Thyroid hormones (T3 and T4) are characterized by active membrane transport, intracellular conversion of T4 into the more active T3, and binding to nuclear receptors.
Regulation of sensitivity
The number of receptors on a cell is not constant—they are continuously synthesized and degraded. The sensitivity of the target cell adapts to the hormone level:
- During a deficiency of the hormone, the number of receptors increases.
- During an excess, it decreases.
Membrane receptors for protein-peptide hormones can undergo internalization: upon binding to a ligand, the receptor is pulled inside the cell and inactivated.
Briefly: answers to topical questions
Which substances act as secondary messengers during hormonal signal transmission?
Specific molecules and ions that amplify the primary signal act as secondary messengers (mediators) inside the cell.
The main second messenger systems include:
- cAMP — cyclic adenosine-3',5'-monophosphate;
- cGMP — cyclic guanosine-3',5'-monophosphate;
- IP₃ — inositol-1,4,5-trisphosphate (a water-soluble substance that enters the cytosol);
- DAG — diacylglycerol (a hydrophobic substance that remains in the membrane);
- Ca²⁺ — calcium ions.
A common property of these systems is the multi-step amplification of the signal, allowing hormones to act effectively even at very low blood concentrations.
What is the specificity of the mechanism of action of thyroid hormones compared to steroid hormones?
The specificity of thyroid hormone action compared to steroid hormones lies in the mode of cell entry, the presence of intracellular conversion, and the characteristics of receptor binding.
| Characteristic | Thyroid hormones | Steroid hormones |
|---|---|---|
| Cell penetration | T3 and T4 are transported in the blood by specific proteins; binding to the receptor and dissociation of the hormone from the transport protein occur at the target cell membrane surface; transport across the membrane is mediated by an active transporter | Steroid hormones, being nonpolar substances, freely cross the cell membrane; there is also evidence of membrane receptors that facilitate hormone recognition and its transport into the cell |
| Intracellular conversion | Inside the cell, T4 is deiodinated to T3, the active form | Such a stage is not specified in sources for steroid hormones |
| Receptor binding | T3 binds to protein nuclear receptors | In the cytoplasm, the hormone binds to a cytosolic receptor required for transporting the hormone into the nucleus; in the nucleus, a hormone-receptor complex forms with the nuclear receptor |
In both cases, the final effect is related to the regulation of transcription and protein synthesis, resulting in altered cell function.
How does the inositol phosphate mechanism of hormonal signal transmission work?
The inositol phosphate signal transduction mechanism is implemented via the activation of membrane Gq-proteins and phospholipase C.
- Initiation — receptors couple with Gq-proteins, which activate the enzyme phospholipase C.
- Hydrolysis — activated phospholipase C hydrolyzes membrane phosphatidylinositol-4,5-bisphosphate.
- Generation of messengers — secondary messengers are formed: water-soluble inositol 1,4,5-trisphosphate (IP₃) and hydrophobic diacylglycerol (DAG).
- Calcium release — IP₃ interacts with calcium channels of the sarcoplasmic or endoplasmic reticulum, causing the release of Ca²⁺ into the cytoplasm.
An increase in cytoplasmic Ca²⁺ concentration triggers a cellular response, such as the formation of a complex with calmodulin and subsequent contraction of vascular smooth muscle cells.
Class and exam questions
Why do protein hormones need membrane receptors?
Protein hormones are hydrophilic and incapable of dissolving in lipids, which is why they cannot cross the cell membrane. They must transmit signals via cell surface receptors.
Which effect occurs faster: from membrane or intracellular receptors?
From membrane receptors. They activate enzymes already existing in the cell, whereas intracellular reception requires time for the synthesis of new proteins.
What are second messengers?
These are intracellular substances (cAMP, cGMP, calcium, etc.) that are formed after the activation of a membrane receptor and transmit the signal further, amplifying it manifold.
What else to study on this topic
- Structure and functions of ionotropic receptors
- Catalytic receptors (tyrosine kinases and guanylyl cyclases)
- Types and functions of G-proteins (Gs, Gi, Gq)
- The role of calcium ions, inositol trisphosphate, and diacylglycerol as second messengers
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