Chemical Classification of Hormones
To understand pharmacokinetics and the pathways of signal transduction, all hormonal compounds are divided into three groups:
- Amino acid derivatives — thyroid hormones.
- Peptide and protein hormones — secreted by the hypothalamus, pituitary gland, pancreas, and parathyroid glands.
- Steroid hormones — produced by the adrenal cortex and sex glands.
Intracellular Localization of Receptors
The location of the target receptor directly depends on the chemical nature of the hormone itself:
- Outer cell membrane — receives signals from peptide structures that cannot independently cross the membrane barrier.
- Intracellular localization (in the cytoplasm) — characteristic of lipophilic steroid hormones that penetrate inside the cell.
Mechanism of Action of Protein and Peptide Hormones
These compounds act without entering the cell, utilizing a second messenger system.
The reaction cascade includes the following steps:
- Stimulation of the membrane receptor.
- Activation of G proteins.
- Increase in the enzymatic activity of adenylate cyclase.
- Accumulation of cyclic adenosine monophosphate (cAMP).
- Activation of cAMP-dependent protein kinases.
The end result of this cascade is a change in channel permeability, ion transport, enhanced secretion, synthesis of substances, and cellular proliferation. A special case is observed with triiodothyronine: in high doses, it increases cAMP levels by inhibiting the enzyme phosphodiesterase.
Genomic Mechanism of Steroid Hormones
Steroids exert their influence directly on the genetic apparatus of the cell. The process occurs in stages:
- Penetration: high lipophilicity allows molecules to easily pass through histohematic barriers and membranes.
- Binding: a steroid–receptor complex with high DNA tropism is formed in the cytoplasm.
- Translocation and nucleus: the complex enters the cell nucleus and binds to DNA.
The result of interaction with the genome is the regulation of transcription — the induction (upregulation) or suppression (downregulation) of the synthesis of specific protein molecules.