Function of steroids
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I now have comprehensive information from authoritative medical textbooks. Here is a detailed answer:
Functions of Steroids
Steroids are a class of lipid-derived hormones synthesized from cholesterol. They are broadly divided into:
- Glucocorticoids (e.g., cortisol)
- Mineralocorticoids (e.g., aldosterone)
- Sex steroids (androgens, estrogens, progestins)
The adrenal gland is a key site of steroid synthesis:
The zona glomerulosa produces aldosterone (mineralocorticoid), zona fasciculata produces cortisol (glucocorticoid), zona reticularis produces androgens (DHEA, androstenedione), and the medulla produces epinephrine.
Mechanism of Action (All Steroids)
Steroids act via a genomic mechanism - unlike peptide hormones which use surface receptors and second messengers, steroids:
- Diffuse across the cell membrane (they are lipid-soluble)
- Bind to intracellular receptor proteins in the cytosol or nucleus (each receptor has 6 domains; hormone binds in domain E near the C-terminus; DNA-binding occurs via zinc fingers in domain C)
- The hormone-receptor complex undergoes a conformational change, dimerizes, and enters the nucleus
- It binds to steroid-responsive elements (SREs) in the 5' region of target genes
- Acts as a transcription factor - regulates mRNA transcription
- New proteins are synthesized that carry out the hormone's physiologic effects
This process is slow (hours vs. minutes for peptide hormones). Steroids also have non-genomic actions that occur rapidly (within 0-3 hours).
- Costanzo Physiology, 7th Ed., p. 410
1. Glucocorticoids (Cortisol)
| Function | Details |
|---|---|
| Glucose metabolism | Enhances gluconeogenesis in the liver; mobilizes amino acids from peripheral tissues; raises blood glucose |
| Protein metabolism | Promotes protein catabolism in muscle, skin, and bone; supplies amino acids for gluconeogenesis |
| Fat metabolism | Stimulates lipolysis; fat redistribution (central obesity in excess) |
| Anti-inflammatory | Inhibits cytokine transcription, downregulates adhesion molecules and MHC expression, modifies leukocyte trafficking |
| Immunosuppressive | Induces lymphocyte apoptosis; suppresses T-cell function and antibody production |
| CNS effects | Influences mood, cognition, and behavior |
| Calcium/bone | Inhibits Ca²⁺ absorption from gut, promotes bone resorption; can cause osteoporosis in excess |
| Stress response | Primary "stress hormone" - activated by ACTH during physiological stress |
| Cardiovascular | Maintains vascular tone and blood pressure; sensitizes vessels to catecholamines |
Deficiency of glucocorticoids = Addison's disease (fatigue, hypotension, hypoglycemia)
Excess = Cushing's syndrome (central obesity, hyperglycemia, immunosuppression)
- Medical Physiology (Boron & Boulpaep), p. 1496
- Goldman-Cecil Medicine
2. Mineralocorticoids (Aldosterone)
| Function | Details |
|---|---|
| Na⁺ reabsorption | Induces synthesis of Na⁺ channels (ENaC) in renal principal cells - promotes Na⁺ (and water) reabsorption |
| K⁺ excretion | Promotes K⁺ secretion into the tubular lumen |
| Blood pressure | By retaining Na⁺ and water, increases blood volume and blood pressure |
| Acid-base | Promotes H⁺ secretion by intercalated cells, contributing to metabolic alkalosis in excess |
Excess aldosterone (hyperaldosteronism) causes hypertension and hypokalemia.
3. Sex Steroids
Androgens (Testosterone, DHEA, Androstenedione)
- Promote development of male secondary sexual characteristics (facial hair, deepening of voice, penile growth, libido)
- Anabolic effects: stimulate skeletal muscle protein synthesis
- Regulate spermatogenesis
- Adrenal androgens drive adrenarche (pubic/axillary hair before puberty)
Estrogens (Estradiol, Estrone)
- Female secondary sex characteristics (breast development, female fat distribution)
- Regulate the menstrual cycle and endometrial growth
- Maintain bone density (inhibit osteoclast activity)
- Cardiovascular protection (increase HDL, decrease LDL)
- Feedback regulation of the hypothalamic-pituitary axis
Progestins (Progesterone)
- Prepares the endometrium for implantation
- Maintains pregnancy (inhibits uterine contractions)
- Thermogenic effect (raises basal body temperature after ovulation)
4. Other Steroids
| Steroid | Function |
|---|---|
| Vitamin D (calcitriol) | Promotes intestinal Ca²⁺ absorption via synthesis of a Ca²⁺-binding protein; regulates bone mineralization |
| Bile acids | Derived from cholesterol; essential for fat digestion and absorption |
| Neurosteroids | Modulate GABA and NMDA receptor activity in the brain |
Summary Table
| Class | Example | Key Function |
|---|---|---|
| Glucocorticoids | Cortisol | Glucose regulation, anti-inflammatory, stress response |
| Mineralocorticoids | Aldosterone | Na⁺/K⁺ balance, blood pressure |
| Androgens | Testosterone | Male sex characteristics, anabolic effects |
| Estrogens | Estradiol | Female sex characteristics, bone, cardiovascular |
| Progestins | Progesterone | Pregnancy maintenance, endometrium |
| Vitamin D | Calcitriol | Ca²⁺ absorption, bone health |
Sources: Costanzo Physiology 7th Ed. | Medical Physiology (Boron & Boulpaep) | Goodman & Gilman's Pharmacological Basis of Therapeutics | Goldman-Cecil Medicine
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