The tally of mammalian hormones extends well beyond the molecules just discussed. What is most puzzling is how the many hormones are interconnected during the control of development. Hormones control gene activities of target cells, and some simple hormone systems act antagonistically (calcitonin-parathormone, insulin-gluca-gon). Yet there has not emerged a clear and complete picture of the overall interactions. Hormones control an incredibly complex array of cellular activities from conception to death.
Mammalian developmental hormones have been studied using a variety of biochemical and physiological experiments: isolation and purification experiments, injection into experimental animals, studies of metabolic disorders in animals, and molecular genetics experiments. Protein structures based upon genetic and biochemical studies are well understood. Steroid hormone structures have been unraveled from studies of cholesterol biochemistry.
Dissections of experimental animals yield intact endocrine glands (such as the thyroid and pancreas) that can be used to show function. Chemical secretions from these glands can be extracted and separated into the various hormone components by several biochemical techniques such as electrophoresis, chromatography, and centrifugation. The isolated hormones can be further purified by rerunning them through these separatory techniques.
Electrophoresis involves the separation of molecules in an electric field based upon their sizes and charges. Large molecules move slowly, whereas small, compact molecules move more quickly. Protein hormones move from the negative pole to the positive pole in electrophoretic gels. Affinity chromatography involves placing membrane hormone receptor proteins on a vertical column containing a porous resin. The specific hormone type that binds to this particular target receptor protein will stick to the resin. Nonbinding hormones will wash through the column. Finally, ultracentrifugation separates molecules based upon size in incredibly high-spinning gravity fields mea suring about 100,000 times the earth's gravity. These three techniques, plus a few others, are very effective in isolating and purifying hormones as well as other important molecules.
Isolated hormones have been injected into experimental organisms and organ extracts, followed by observation and recording of the animal's physiological responses. For example, injection of vasopressin reduces an animal's urine output while simultaneously producing a slight blood pressure rise. Injection of insulin lowers blood sugar levels, which is why diabetics are prescribed insulin. Such experiments require the use of experimental animals, and extracts from these animals, which has sparked considerable controversy and debate concerning animal rights. These studies are important in understanding the physiology of the human body.
Genetic studies such as cloning and DNA sequencing have identified genes that may encode other developmental hormones. Discovery of the homeobox within the genes of all mammals indicates that there are some proteins (hormones) that control basic pattern development in mammals during early embryonic development. Some researchers believe that there may be certain hormones that accelerate aging and cause death in later life.
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