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 cross the lipid bilayer of the membrane on their own.
There are two main groups:
- Ionotropic receptors. These are proteins that act as both a receptor and an ion channel. When a ligand binds to them, the channel opens immediately, causing an ion current (e.g., nicotinic acetylcholine receptors).
- Metabotropic receptors. Their activation triggers a cascade of second messengers (mediators) inside the cell.
Metabotropic receptors are further divided into catalytic receptors (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 use G proteins to transmit signals into the cell.
Second Messenger Systems
During 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 affects the enzyme adenylyl cyclase.
- Adenylyl cyclase converts ATP into the second messenger — cAMP.
- cAMP activates cAMP-dependent protein kinases.
- Protein kinases phosphorylate proteins (usually enzymes), altering their activity and eliciting a cellular response.
The main property of such cascades is the 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 unfolds as follows:
- The hormone detaches 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 (mRNA synthesis) is activated, 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.
Sensitivity Regulation
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 hormone deficiency, the number of receptors increases.
- During hormone excess, it decreases.
Membrane receptors of protein-peptide hormones can undergo internalization: after binding to a ligand, the receptor is pulled inside the cell and inactivated.