Hormones - Anatomy & Physiology
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Hormones are signalling molecules produced by endocrine cells which convey information to target cells.
When a hormone reaches a target cell, it binds to a receptor to initiate a response.
Classifications of Hormones
Hormones can be classified by various means, including by their structure and function.
Traditionally there are three classifications of hormones:
- Classical Hormones: secreted from endocrine cells directly into interstitial fluid. These diffuse into the bloodstream to be distributed to all parts of the body served by the circulatory system.
- Neurohormones: synthesized in neuroendocrine cells and secreted from nerve terminals. These diffuse into blood vessels for transportation around the body.
- Local Hormones: these are secreted into the interstitial fluid and act locally, on neighbouring cells (paracrine action) or on the cell which secreted them (autocrine action).
Additionally, hormones can be classified by their structure:
- Amino Acid Derivatives: these are derived from the amino acid Tyrosine. Examples include Catecholamines and Thyroid Hormones.
- Fatty Acid Compounds: these are derived from a polyunsaturated fatty acid pre-cursor, usually arachidonic acid. Hormones with this structure form the group Eicosenoids, and are important in imflammatory processes.
- Peptide Hormones: these vary from small peptides to long chain proteins. They are synthesized via transcription and translation pathways within cells, and may be derived from prohormones. They are secreted out of the endocrine cell by exocytosis. Examples include Insulin.
- Steroid Hormones: these lipid soluble hormones are derived from cholesterol. They are synthesised and secreted as needed; there is no capacity for storage. Examples include Cortisol, Androgens and Calcitriol.
Hormones which are not bound in the blood can be defined as 'active' and are able to bind to the target cell to initiate a response.
Water Soluble Hormones
Water soluble hormones are freely transported within the blood. In order to enter a cell they must bind to a membrane receptor as they cannot diffuse through the lipid bilayer.
Lipid Soluble Hormones
Lipid soluble hormones require a binding protein, usually of a polar nature in order to be transported within the bloodstream. They can freely diffuse through the target cell membranes in order to initiate a response.
Hormone Binding Proteins
Binding proteins, or globulins, are transporters of lipid soluble hormones. They can be specific to a particular hormone, or non specific, having the ability to carry many types of hormone.
Examples of Hormone Binding Proteins: Specific:
- Cortisol Binding Protein
- Vitamin D Binding Globulin
- Thyroid Binding Globulin (carries T3 and T4)
- Albumin (carries all types of steroids)
- Prealbumin (carries T3 and T4)
Transport proteins are synthesised and degraded within the liver.
Hormone Binding Protein Functions
- Hormone 'reservoir' - an equilibrium exists between the concentration of free hormone and the concentration of bound hormone. As free hormones bind to their receptors, the binding proteins release some of their load to maintain the plasma concentration of the free hormone.
- Hormone level 'buffer' - Normal function involves use of only 50% of the binding capacity of hormone binding proteins. Thus they have the capacity to absorb short term peaks and troughs in synthesis of the hormone which they can carry.
- Reduce hormone loss - hormones bound to a protein cannot cross the glomerulus of the kidney, thus cannot be excreted in the same way as free hormones.
These functions result in a maintained concentration of free hormones within the bloodstream.
Plasma concentration of hormones in the blood is a net result of:
- Rate of secretion into the blood
- Rate of removal (elimination) from the blood
Elimination mechanisms include:
- Enzymatic degradation
- Within target cells after receptor binding
- Via the liver and kidneys.
Factors affecting Hormone Response
- Variation by Tissue - Different tissues vary in their response to a particular hormone.
- Variation by time - the initial response to a hormone may differ to the delayed response. This depends on the animal's age, species, and by the hormone involved.
- Variation by dose - Hormone levels outside the normal physiological range may elicit different responses to those within the normal range.
- Status of target tissue - With overstimulation, target tissues hypertrophy, leading to increased functional tissue and an exaggerated response. Understimulation of target tissues leads to atrophy, resulting in less functional tissue and an inadequate response.
Chemical Structure of Hormones
The chemical structure of TRH (Thyrotropin Releasing Hormone), GnRH (Gonadotropin Releasing Hormone), Somatostatin and Dopamine, which are secreted by the hypothalamus, is similar in all mammals. On the contrary, CRH (Corticotropin Releasing Hormone) and GHRH (Growth Hormone Releasing Hormone), vary in their amino acid sequence from mammal to mammal. TRH is a tripeptide, GnRH is a decapeptide and dopamine is a catecholamine. CRH and GHRH have 41 and 44 amino acids respectively.
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