Mechanisms of hormone secretion
The method of hormone release from an endocrine cell strictly depends on its chemical nature.
- Protein-peptide hormones are stored inside cells in specialized secretory granules. Their release occurs through the mechanism of exocytosis. Secretion can be basal (continuous) or stimulated (in response to nervous or humoral signals). With adequate stimulation, exocytosis ensures the release of exactly the amount of substance needed in a given situation.
- Catecholamines (synthesized in the adrenal medulla) are packaged into vesicles as they are formed and released similarly to peptides—via exocytosis.
- Thyroid hormones are stored in the glandular tissue as specific droplet inclusions.
- Steroid hormones fundamentally differ in that they do not accumulate in cells. When a specific stimulus arrives, the synthesis of their active forms is induced, after which the molecules immediately diffuse through the cell membrane directly into the internal environment.
Biological rhythms of secretion
For the vast majority of hormones, there is no stable basal level—their concentration constantly changes, obeying biological rhythms.
- Episodic (pulsatile) secretion: manifests as short-term peak releases at intervals ranging from a few minutes to a few hours. A classic example is the pulsatile secretion of gonadoliberin, the peaks of which recur approximately every 90 minutes.
- Diurnal (circadian) rhythmicity: typical of growth hormone (somatotropin), melatonin, and cortisol. The regulator is the hypothalamus, specifically its suprachiasmatic nuclei. For instance, the maximum plasma concentration of cortisol is recorded in the morning hours. This is due to the fact that 2–3 hours before this peak, the level of adrenocorticotropic hormone (ACTH) rises, stimulating the adrenal glands to produce cortisol.
- Cyclic (multiday) rhythmicity: has a period of about 28 days and is pronounced in the ovarian-menstrual cycle of women. It encompasses fluctuations in gonadotropins, estrogen, progesterone, and inhibins.
Circulation, metabolism, and elimination
Once in the blood, hormones are transported in three main forms:
- Free form: predominates for protein-peptide hormones.
- Complex with transport proteins: characteristic of steroids and thyroid hormones.
- Adsorption onto the membranes of formed blood elements.
Important rule: only the freely circulating hormone possesses biological activity. The protein-bound fraction acts as a reserve pool. A dynamic equilibrium is constantly maintained between these two forms.
The rate of substance removal from plasma is described by the half-life ($T_{1/2}$)—the period during which 50% of the biologically active hormone is eliminated. Protein-peptide hormones are rapidly degraded by enzymes (peptidases), so their $T_{1/2}$ ranges from minutes to hours. Steroids and thyroid hormones are protected by binding to transport proteins: the half-life of steroids is hours, and that of thyroid hormones is days.
The removal of hormones occurs through their metabolism, internalization by target cells, and excretion. Up to 90% of hormones are eliminated from the body in an inactivated state. The main burden is borne by the kidneys (up to 80% of excretion), with participation from the liver (release with bile into the GI tract), sweat glands, and salivary glands.
Briefly: answers to topical questions
Which specific transport proteins bind steroid and thyroid hormones in plasma?
According to sources, the transport/binding of steroid and thyroid hormones in plasma involves:
- Albumins — transport steroid hormones, thyroxine, and triiodothyronine.
- Globulins — carry thyroid hormones (T3 and T4); for T4, binding to albumin and globulin is indicated.
- Transcortin — transports cortisol.
- Sex hormone-binding globulin (SHBG) — binds sex hormones.
Which specific enzymes (peptidases) are involved in the degradation of protein-peptide hormones in the blood and tissues?
Protein-peptide hormones are degraded by enzymes of the peptidase group. Among specific representatives, the materials mention only one enzyme:
- Dipeptidyl peptidase-4 (DPP-4) — carries out the enzymatic degradation of endogenous incretins (glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide).
In general, protein-peptide hormones are rapidly degraded by peptidases, so their half-life ranges from several minutes to several hours.
Peptides are like groceries in a fridge (stored in granules and dispensed in portions). Steroids are like fresh hot pastries (not accumulated, but immediately "sold" across the membrane).
Class and exam questions
Can a hormone in a protein complex exert a biological effect?
No, only the freely circulating hormone has a physiological effect. The bound form serves as a dynamic reserve protecting the hormone from rapid degradation.
Why do peptide hormones degrade faster than steroids?
Peptide hormones are present in the blood predominantly in free form and are rapidly cleaved by peptidase enzymes. Steroids, however, circulate in complexes with transport proteins, which protects them and increases their half-life.
What role does ACTH play in the circadian rhythm of cortisol?
ACTH is a tropic pituitary hormone. Its level rises 2–3 hours before the morning cortisol peak, triggering the induction of synthesis and secretion of this steroid in the adrenal glands.
What else to study on this topic
- Mechanisms of enzymatic modification of hormone precursor molecules
- The process of hormone internalization by a target cell
- The role of the suprachiasmatic nuclei in hypothalamic rhythm regulation
- Dynamic equilibrium between free and bound hormone fractions
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