Glucocorticoids: Systemic Effects on the Body
These hormones possess a wide spectrum of physiological activity, affecting virtually all types of metabolism and organ systems.
Metabolic Effects The effect on lipid metabolism manifests as a specific redistribution of adipose tissue: in synergy with insulin, glucocorticoids stimulate lipogenesis predominantly in the upper half of the trunk, face, and neck. Regarding mineral metabolism, at high concentrations these hormones can bind to mineralocorticoid receptors. This leads to the retention of sodium ($Na^+$) and water ($H_2O$) in the renal tubules, as well as enhanced potassium ($K^+$) loss.
Immune System and Hematopoiesis Glucocorticoids exert a marked effect on the cellular composition of the blood, reducing the absolute number of lymphocytes, eosinophils, and basophils. They act as potent regulators of immunity:
- They suppress cell-mediated immune responses.
- With prolonged exposure, they suppress humoral immunity.
- By enhancing protein breakdown (catabolism) in lymphoid tissue and generally suppressing the immune response, they provide a powerful anti-allergic effect.
- They inhibit the development of inflammatory processes (anti-inflammatory action).
Nervous, Endocrine Systems, and Stress These hormones increase central nervous system excitability and sharpen sensory sensitivity. In the adrenal medulla, they stimulate the production of catecholamines (mainly epinephrine) and simultaneously increase the sensitivity of adrenergic receptors to these neurotransmitters.
Glucocorticoids play a key role in mounting a full stress response. They mobilize physiological functions, increase energy reserves, maintain adequate cardiovascular regulation, and ultimately multiply the body's overall resistance to stressors.
Mineralocorticoids: Aldosterone and Water-Electrolyte Balance
The main representative of this group is aldosterone, synthesized by the cells of the adrenal cortex. Its primary physiological task is to maintain electrolyte balance in the body fluids and regulate systemic hemodynamics as a crucial link in the renin-angiotensin-aldosterone system (RAAS).
Mechanism of Action and Pharmacokinetics Like all steroids, aldosterone crosses the cell membrane and binds to intracellular (nuclear) receptors. The resulting hormone-receptor complex initiates the transcription of specific genes responsible for synthesizing transport proteins and enzymes. The hormone circulates in the blood primarily unbound, and its half-life is extremely short (less than 15 minutes). Inactivation occurs in the liver tissue, and metabolite excretion occurs via the kidneys.
Physiological Effects The primary target organs for aldosterone are the kidneys, specifically the distal convoluted tubules and the initial segments of the collecting ducts. In these structures, the hormone:
- Significantly increases the reabsorption of sodium, chloride, and bicarbonate ions.
- Follows sodium along an osmotic gradient to enhance water reabsorption.
- Increases fluid volume, leading to an expanded extracellular fluid volume (ECFV) and a concurrent rise in blood pressure.
- Simultaneously stimulates the excretion of hydrogen ($H^+$) and potassium ($K^+$) ions.
A similar effect on electrolyte metabolism is observed in the ducts of the salivary and sweat glands. Additionally, aldosterone possesses pro-inflammatory potential: it promotes fluid extravasation from the vascular bed into surrounding tissues, leading to edema formation.
Regulation of Secretion Aldosterone production is tightly controlled by several factors:
- Main stimulator: Angiotensin II.
- RAAS activation triggers: Drop in blood pressure, decreased ECFV, hyponatremia, and elevated sodium concentration in distal tubular fluid.
- Feedback: Hyperkalemia directly stimulates hormone release, whereas hypokalemia inhibits it.
- Additional factors: ACTH (adrenocorticotropic hormone) transiently increases secretion (especially during stress).
- Inhibitor: Atrial natriuretic peptide (ANP) blocks aldosterone production.
Sex Hormones of the Adrenal Cortex
The adrenal cortex also secretes androgens. Their characteristic feature is the capacity for peripheral conversion: once in target tissues, they are partially transformed into testosterone and estrogens by local enzymes.
Physiological Significance Adrenal androgens play a critically important role in childhood, when the intrinsic endocrine function of the gonads is not yet fully developed. In women, they constitute the bulk of the androgen pool. With aging, the synthesis of these steroids in the adrenal glands decreases by approximately one-third.
Main Functions of Androgens:
- Ensuring the formation and development of secondary sexual characteristics.
- Marked anabolic action (direct stimulation of protein synthesis).
- Stimulation of growth plate closure (ossification of the epiphyseal cartilage of tubular bones).
Androgen secretion is controlled by adrenocorticotropic hormone (ACTH) via the classic negative feedback loop. It is characterized by a strict circadian rhythm with peak values in the morning (6:00 AM to 8:00 AM) and a trough in the evening (7:00 PM to 8:00 PM).