Hormones

Hormones are chemical signalling molecules which are released from Endocrine glands; other tissues, such as the Kidney and heart muscle, apart from glands can produce hormones, however the effects of these hormones are usually more local than those of hormones produced by Endocrine glands. Endocrine tissues/glands which both males and females possess are the pineal gland, hypothalamus, pituitary gland, thyroid gland, parathyroid glands, thymus, adrenal glands and islets of Langerhans in the Pancreas; males also have Testes which produce Testosterone, while females have Ovaries which produce Oestrogen. Each of the Endocrine glands produces and secretes hormones; hormones are secreted from their glands into the blood stream and, generally, act on distant target cells which possess specific Receptor molecules to regulate cellular function. Receptors can be located either on the target cell Plasma membrane or inside the cell depending on the type of hormone .

There are three different types of hormones:

• Steroid hormones -- derived from cholesterol, steroid hormones are small, Hydrophobic, lipiphilic molecules which can diffuse across cell the Plasma membrane. Steroid hormones are not released prior to release; following synthesis, steroid hormones are immediately released. The Receptors for steroid hormones exist in the Cytoplasm or Nucleus of the target cell; there is no need for Plasma membrane receptors as steroid hormones are able to diffuse across the Plasma membrane to reach internal receptor molecules. An example of a steroid hormone is Oestrogen.
• Peptide hormones -- chains of Amino acid residues encoded by Genes; peptide hormones are long molecules which are synthesised then stored prior to release in a less active form (preprohormones). Peptide hormones are hydrophilic and lipiphobic therefore, cannot cross the Plasma membrane; as they cannot cross the membrane, receptors for peptide hormones must be placed on the cell surface membrane. An example of a peptide hormone is Insulin which is synthesised in Beta cells of the Islets of Langerhans in the Pancreas.
• Amino acid hormones -- mostly synthesised from tyrosine, amino acid hormones are stored for instant release upon reception of a signal. Depending on the molecule, amino acid hormones can act as either a steroid or peptide hormone; for example, thyroid hormone is an amino acid hormone which acts like a steroid, while epinephrine, which is also an amino acid hormone, acts as a peptide hormone.

Hormones can be released by various mechanisms:

• Continuous -- hormones are released continuously into the blood stream from their endocrine glands and its effects are controlled by presence/absence of its receptor on its target cells rather than up- or down-regulating hormone release. For example, Thyroid hormone under the control of TSH.
• Pulsatile -- hormones release is either on or off; release is up- or down-regulated according to the requirements of the individual. For example, Growth Hormone Releasing Hormone which regulates the release of Growth Hormone.
• Circadian -- the hormone is released as a step in a cycle. For example, the release of melatonin at night due to a decrease in light intensity creating the feeling of fatigue.
• Exocytosis on stimulus -- for example, the release of Insulin as a result of the detection of an increase in blood Glucose levels.

Hormone effects must be controlled in accordance with the individual's requirements; hormones can be controlled by these means:

• Modification -- adding or removing groups from the hormone molecule will up- or down-regulate its effects. Modification increases or decreases the hormone's activity according to requirements.
• Degradation -- the hormone is broken down and excreted, therefore removing its active effects from the system.
• Receptor down-regulation -- receptors on the target cells of the hormone can be internalised or brokwn down so that the hormone has nowhere to bind; if the hormone cannot bind to a receptor on the target cell, it cannot give rise to the effects it would normally cause.
• Termination of intracellular effects -- for exmaple, enzymes/proteins involved in the steps following the binding of the hormone to its receptor can be inactivated, therefore, eliminating the hormone's effects.
• Negative feedback -- action of the hormone shuts down further production of the hormone. For example, Insulin is produced when blood Glucose levels are deemed too high; Insulin acts to reduce blood glucose levels, therefore reducing the production of more Insulin.