Mechanisms of Hormone Secretion
The mechanism of hormone release from an endocrine cell strictly depends on its chemical nature.
- Protein and peptide hormones are stored intracellularly in specialized secretory vesicles. Their release occurs via exocytosis. Secretion can be basal (continuous) or stimulated (in response to neural or humoral signals). Upon adequate stimulation, exocytosis ensures the release of precisely 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 within the glandular tissue as specific droplet inclusions.
- Steroid hormones fundamentally differ in that they are not stored within cells. Upon receiving a specific stimulus, the synthesis of their active forms is induced, after which the molecules immediately diffuse across 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 in accordance with biological rhythms.
- Episodic (pulsatile) secretion: manifests as short-term peak releases with intervals ranging from several minutes to several hours. A classic example is the pulsatile secretion of gonadotropin-releasing hormone (GnRH), with peaks repeating approximately every 90 minutes.
- Circadian rhythmicity: typical of growth hormone (GH), melatonin, and cortisol. The hypothalamus acts as the regulator, specifically its suprachiasmatic nuclei. For example, the peak plasma cortisol concentration is recorded in the morning hours. This is because 2–3 hours prior to this peak, the level of adrenocorticotropic hormone (ACTH) rises, stimulating the adrenal glands to produce cortisol.
- Cyclical (multiday) rhythmicity: has a period of about 28 days and is prominent in the female ovarian-menstrual cycle. It encompasses fluctuations in gonadotropins, estrogens, progesterone, and inhibins.
Circulation, Metabolism, and Clearance
Upon entering the blood, hormones are transported in three main forms:
- Free form: predominates for protein-peptide hormones.
- Complex with carrier 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 removal of a substance 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 carrier protein binding: the half-life of steroids is measured in hours, and that of thyroid hormones in days.
Removal of hormones occurs through metabolism, internalization by target cells, and excretion. Up to 90% of hormones are eliminated from the body in an inactivated state. The kidneys bear the primary load (up to 80% of excretion), with participation also from the liver (excretion via bile into the GI tract), sweat glands, and salivary glands.