Overview
The adrenal cortex makes steroid hormones from cholesterol: mineralocorticoids, glucocorticoids and adrenal androgens. Glucocorticoid output is controlled by the hypothalamic-pituitary-adrenal axis, and too much or too little cortisol produces characteristic clinical syndromes.
General characteristics of steroid hormones
- Metabolism refers to the sum of all chemical reactions occurring within an organism to maintain life.
- It includes anabolism (building molecules) and catabolism (breaking down molecules).
- Metabolic pathways are tightly regulated to ensure energy balance and homeostasis.
- Integration involves the coordination of various organs, tissues, and cells to respond to nutrient availability and energy demands.
- Hormones: Chemical messengers that regulate metabolism by signalling between organs and tissues.
- Enzymes: Catalytic proteins that control the rate of metabolic reactions.
- Substrate availability: Nutrient levels in the bloodstream influence metabolic pathways.
Fasting vs. Feeding States:
- Fasting: Body switches to catabolic pathways to generate energy (lipolysis).
- Feeding: Anabolic pathways are activated to store excess nutrients (lipogenesis).
Exercise:
- Increased energy demand triggers catabolic pathways to release energy (glycolysis, \beta-oxidation).
- Hormones are signalling molecules secreted by glands into the bloodstream to regulate physiological processes in distant target cells.
- Peptide/Protein Hormones:
- Composed of amino acids.
- Examples: Insulin, Growth hormone, Glucagon.
- Steroid Hormones:
- Derived from cholesterol.
- Examples: Estrogen, Testosterone, Cortisol.
- Amino Acid-Derived Hormones:
- Derived from amino acids.
- Examples: Thyroxine (derived from tyrosine), Epinephrine (derived from tyrosine).
- Endocrine: Hormones travel through the bloodstream to target cells.
- Paracrine: Hormones act locally on nearby cells.
- Autocrine: Hormones act on the same cell that produced them.
- Feedback mechanisms involving hormones, neurotransmitters, and other factors regulate hormone secretion.
- Examples: Negative feedback loops maintain hormone levels within a narrow range.
Steroid Hormone Structure:
- Steroid hormones are lipid-soluble molecules derived from cholesterol.
- Consist of four interconnected rings.
Intracellular Receptors:
- Steroid hormones diffuse across the cell membrane and bind to intracellular receptors (usually in the cytoplasm or nucleus).
- Hormone-receptor complex translocates to the nucleus.
- Complex binds to specific DNA sequences (hormone response elements) and regulates gene transcription.
Gene Transcription:
- Activation or repression of target genes leads to changes in mRNA synthesis.
- mRNA is translated into proteins that mediate the physiological response to the hormone.
- Estrogen: Regulates female reproductive functions, bone density, and cardiovascular health.
- Testosterone: Controls male reproductive functions, muscle mass, and secondary sexual characteristics.
- Cortisol: Modulates metabolism, immune response, and stress adaptation.
- Controlled by feedback mechanisms involving the hypothalamus, pituitary gland, and target tissues.
- Stimulated by factors such as stress, circadian rhythms, and other hormones.