Endocrinology

Endocrinology is a branch of medicine dealing with disorders of

Hormones

The endocrine system is comprised of several glands located in diverse areas of the body. Morphologically they are noteworthy because they secrete directly into the bloodstream rather than into a duct system. The hormones produced by these glands can have different functions and modes of actions. One hormone can have multiple effects and target organs and, likewise, one physiological event or target organ can be affected by more than one hormone.

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In 1902 Bayliss and Starling performed an experiment in which they observed that acid instilled into the duodenum caused the pancreas to begin secretion, even after they had removed all nervous contact between the two. They also discovered the same response could be engendered by injecting jejunal mucosa. This demonstrated that some other factor, one that is inherent in the mucosa, was causing a physiological response, and not the nervous system which, at the time, was thought to be the only means of bodily communication. They termed the substance responsible for this chemical reflex secretin and coined the term hormone for chemicals that act in this manner. As part of this definition they specified that a chemical must be produced by an organ; be released (in small amounts) into the bloodstream; be transported to a distant organ and exert its specific function in order to be classified as a hormone. Although this definition applies to almost all ?classical? hormones there also exists paracrine mechanisms (a chemical communication between neighbouring cells within a tissue or organ), autocrine signals (a chemical that acts on the same cell) and intracrine signals (a chemical that acts within the same cell) (Nussey and Whitehead, 2001). A neuroendocrine signal is a type of ?classical? hormone and is released into the bloodstream by a neurosecretory cell upon excited by a nervous signal.

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Griffin and Ojeda (2000) identify three different classes of hormone based on their chemical composition:

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Amines, such as norepinephrine, epinephrine and dopamine, are derived single amino acids, in this case tyrosine. Thyroid hormones like 3,5,3?-triiodothyronine (T3) and 3,5,3?,5?-tetraiodothyronine (thyroxine, T4) make up a subset of this class as they derive from the combination of two iodinated tyrosine amino acid residues.

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Peptide and Protein hormone are comprised of between three (in the case of thryotropin-releasing hormone (glutamic acid?histidine?proline)) and over 200 (in the case of follicle-stimulating hormone) amino acid residues and can have molecular weights as large as 30,000.

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Steroid hormones are derivatives of cholesterol and are subdivided into those with an intact steroid nucleus (gonadal and adrenal steroids) and those with a broken steroid nucles (Vitamin D).

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Hormones mediate their effects by binding to specific and unique receptors in the target organ or organs, although the response following such recognition varies commensurate with the wide array of differing hormone functions. As Baulieu (1990) notes, though, a receptor consists of at least two basic constituents, to wit, a recognition site to which the hormone binds and an effector (or executive) site, which precipitates the modification of cellular function; between these two entities is a mechanism of transduction in which hormone binding induces allosteric modification that, in turn, produces the appropriate response.

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