Physiological role

Such branching into multiple messages that act on multiple targets is not uncommon with the arachidonic acid cascade, as it is also illustrated by our next example the participation of arachidonate in the electrophysiological effects of the peptide, somatostatin. Somatostatin modulates the excitability of hippocampal 

Possible role of arachidonate in the amplification of insulin secretion from pancreatic islets. Intracellular glucose metabolism is thought to be linked to the stimulation of both calcium channels, which results in insulin secretion, and phospholipase A2 activity, which increases the levels of non-esterified arachidonate. Arachidonate may in turn enhance insulin secretion by altering the voltage-dependence of calcium channels. In addition, arachidonate is converted into cyclooxygenase metabolites which may exit the cell, and modulate insulin secretion by binding to cell membrane receptors on adjacent p cells. Based on results published on refs. 27-32. 

Hamilton RJ, Hamilton S. Lipid Analysis. A Practical Approach. Oxford, University Press, 1992. 

Christie WW. Gas Chromatography and Lipids. A Practical Guide. Dundee, The Oily Press, 1989. 

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Histamine in the Cardiovascular System. It has been known for many years that histamine is present in sympathetic nerves and has a distribution within the heart that parallels that of norepinephrine (see Epinephrine and norepinephrine). A physiological role for cardiac histamine as a modulator of sympathetic responses is highly plausible (15). A pool of histamine in rat heart located neither in mast cells nor in sympathetic nerves has been demonstrated. The turnover of this metaboHcaHy active pool of histamine appears to be maintained by normal sympathetic activity. 

Antibodies have been generated which produce immunoassays that discriminate between GH-V and GH-N. These assay systems have shown that the secretion of GH-V becomes elevated at about three weeks of pregnancy and increases to approximately 15 ng/mL near term (8). The physiological role of GH-V is uncertain. Genetic deficiency of GH-V does not adversely affect pregnancy or fetal development (9). GH-V is a potent growth-stimulator but possesses considerably less lactogenic activity than GH-N (10). There are no clinical appHcations (ca 1993) for GH-V. 

Some elements found in body tissues have no apparent physiological role, but have not been shown to be toxic. Examples are mbidium, strontium, titanium, niobium, germanium, and lanthanum. Other elements are toxic when found in greater than trace amounts, and sometimes in trace amounts. These latter elements include arsenic, mercury, lead, cadmium, silver, zirconium, beryUium, and thallium. Numerous other elements are used in medicine in nonnutrient roles. These include lithium, bismuth, antimony, bromine, platinum, and gold (Eig. 1). The interactions of mineral nutrients with... 

Insufficient zinc results in slowed growth, delayed wound healing, poor appetite, mental lethargy, and sexual immaturity and it interferes with the immune response. The main function of zinc in metaboHsm is enzymatic and there is evidence of other physiologic roles, eg, in stabilization of membrane stmcture (57). 

Physiological Role of Citric Acid. Citric acid occurs ia the terminal oxidative metabolic system of virtually all organisms. This oxidative metabohc system (Fig. 2), variously called the Krebs cycle (for its discoverer, H. A. Krebs), the tricarboxyUc acid cycle, or the citric acid cycle, is a metaboHc cycle involving the conversion of carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organism s growth, movement, luminescence, chemosynthesis, and reproduction. The cycle also provides the carbon-containing materials from which cells synthesize amino acids and fats. Many yeasts, molds, and bacteria conduct the citric acid cycle, and can be selected for thek abiUty to maximize citric acid production in the process. This is the basis for the efficient commercial fermentation processes used today to produce citric acid. 

The variation ia water solubiUty among polysaccharides results ia varied physiological roles. Plant ceU-waH polysaccharides and lignin provide iasoluble dietary fiber (IDF) nondigestible storage polysaccharides, some pectic polysaccharides, and most of the functional additives contribute soluble dietary fiber (SDF). 

Several physicochemical properties of dietary fiber contribute to its physiological role. Water-holding capacity, ion-exchange capacity, solution viscosity, density, and molecular kiteractions are characteristics determined by the chemical stmcture of the component polysaccharides, thek crystallinity, and surface area. 

Nitric oxide (NO) is a gaseous cellular messenger that transmits information between cells and within cells. In spite of its physiological role, NO is also a reactive species which is capable of reacting with biological molecules, and... 

Production of morphinone as a metabolite of morphine and its physiological role 99YZ249. 

Epinephrine itself does find some use in clinical medicine. The drug is used in order to increase blood pressure in cases of circulatory collapse, and to relax the bronchial muscle in acute asthma and in anaphylactic reactions. These activities follow directly from the agent s physiologic role. The biogenetic precursor of epinephrine, norepinephrine, has activity in its own right as a mediator of sympathetic nerve action. (An apocryphal story has it that the term nor is derived from a label seen on a bottle of a key primary amine in a laboratory in Germany N ohne... 

The following chapters will be devoted to the production of j8-poly(L-malic acid) or its salt by fermentation, its Isolation, and physico-chemical characterization. The biosynthesis, degradation, and presumed physiological role will be also considered. 

In this type of reaction an antigen elicits the generation of cytotoxic T-lymphocytes ( immune defense). Cytotoxic T-lymphocytes (Tc) destroy antigen bearing cells by inducing apoptosis. This reaction can be viewed as the cellular counterpart to the humoral Type II reactions. They play an important physiological role in the defense of viruses, and can become allergic reactions under the same conditions as described for Type II reactions. 

Knocking out genes and identification of mutations in the human genes provide information on the role of AQPs in normal physiology. The lack of some AQPs directly results in a disease phenotype, while the physiological role of many becomes clear when the putative function is challenged. 

Guldberg HC, Marsden CA (1975) Catechol-O-methyl transferase Pharmacological aspects and physiological role. Pharmacol Rev 27 135-206... 

The physiological roles of these channels are apparent from human diseases or mouse models in which these genes are disiupted. 

In conclusion, the discovery of incretins has now come to an end in terms of therapeutic strategy of metabolic diseases since new medicines will be put on the market these coming years. However, a lot more needs to be done with regard to the physiological role of the hormones. Whereas the major filed of investigation for GLP-1 now relates to the effect of the peptide on the central nervous system and on (3-cell proliferation, for GIP it remains to understand its role in the numerous tissues where the corresponding receptor is expressed. 

Transgenic animal models with spontaneous or induced receptor gene defects have been instrumental in elucidating the physiological roles of the LDL receptor gene family. In addition, a number of human diseases have been identified that are caused by sporadic or inherited forms of receptor deficiency (Table 1). 

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