Hormones Hormones
Introduction
A hormone is a chemical that acts as a messenger transmitting a signal from one cell to another. When it binds to another cell which is the target of the message, the hormone can alter several aspects of cell function, including cell growth, metabolism, or other function. Hormones can be classified according to chemical composition, solubility properties, location of receptors, and the nature of the signal used to mediate hormonal action within the cell. Hormones that bind to the surfaces of cells communicate with intracellular metabolic processes through intermediary molecules called second messengers (the hormone itself is the first messenger), which are generated as a consequence of the ligandreceptor interaction. The second messenger concept arose from an observation that epinephrine binds to the plasma membrane of certain cells and increasesintracellular cAMP. This was followed by a series of experiments in which cAMP was found to mediate the effects of many hormones. To date, only one hormone, atrial natriuretic factor (ANF), uses cGMP as its second messenger.
Objectives
explain the characterizing hormone
describe functional importance of hormones
Characterizing
Hormone The first way of characterizing a hormone is by looking at the distance over which the hormone acts. Hormones can be classified on three primary ways as following:
1. Autocrine: An autocrine hormone is one that acts on the same cell that released it.
2. Paracrine: A paracrine hormone is one that acts on cells which are nearby relative to the cell which released it. An example of paracrine hormones includes growth factors, which are proteins that stimulate cellular proliferation and differentiation. Specifically, consider the binding of white blood cells to T cells. When the white blood cell binds to a T cell, it releases a protein growth factor called interleukin-1. This causes the T cell to proliferate and differentiate.
3.Endocrine: An endocrine hormone is one that is released into the bloodstream by endocrine glands. The receptor cells are distant from the source. An example of an endocrine hormone is insulin, which is released by the pancreas into the bloodstream where it regulates glucose uptake by liver and muscle cells.
There are three major classifications one should be aware of:
4. Steroids: Steroid hormones are for the most part derivatives of cholesterol.
5. Amino acid derivatives: Several hormones (and neurotransmitters) are derived from amino acids.
6. Polypeptides: Many hormones are chains of amino acids.
