Messengers between cells

Each component of blood has a function ia the body. Red cells transport oxygen and carbon dioxide between the lungs and cells ia the tissues. White cells function as defense of the body. Platelets are important for hemostasis, ie, the maintenance of vascular iategrity. Plasma, an aqueous solution containing various proteias and fatty acids, transports cells, food, and hormones throughout the body. Some proteias ia plasma play a role ia clotting, others are messengers between cells. 

Cytokines. Nonspecific water-soluble glycoproteins with a short half-life produced and secreted abruptly by white blood cells in response to an external stimulus, and which act as chemical messengers between cells. 

The Production of Chemical Messengers between Cells in Organs. 345... 

But in reality, chemokines, the proteins that are the chemical messengers between cells and the immune system, attracted and retained the attention of biologists long before this time. Actually, the first chemokine, human monocyte-derived neutrophil chemotactic factor or interleukin 8 (IL-8), was identified in 1987 (2). Since then, scores of chemokines and their receptors have been identified, and we know they are prominent players in lung defense against infection. 

Intracellular metabolic activity may be modulated in response to extracellular conditions by hormones. Hormones are substances which act as chemical messengers between cells in different locations to alter the activity of the recipient cell. Three classes of hormones have been identified protein (polypeptide) hormones, steroid hormones and tyrosine-derivative hormones of which there are two distinct groups, the catecholamines and the thyroid hormones. The catecholamine hormones (adrenaline and noradrenaline) are produced by and secreted from the adrenal medulla. The thyroid hormones (thyroxine and triiodothyronine) are elaborated by and secreted from the thyroid gland. 

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... 

Chemokines are small chemotactic cytokines that act as important messenger molecules between cells of the immune system. Chemokines produce their effects by activating a family of G-protein-coupled receptors. Chemokine receptors are all seven-transmembrane glycoproteins that are structurally related. They may be characterized into those that bind to specific ligands, or those that bind several chemokine ligands. There are also virally encoded (viral) chemokine receptors that represent shared receptors that have been transduced into the viral genome during evolutionary history (Premack and SchaU 1996). 

A hormone is a chemical that transfers information and instructions between cells in animals and plants. They are often described as the body s chemical messengers , but they also regulate growth and development, control the function of various tissues, support reproductive functions, and regulate metabolism (i.e. the process used to break down food to create energy). 

Second-messenger-dependent kinases, such as cAMP-dependent protein kinase A (PKA) and protein kinase C (PKC), are most often implicated in heterologous desensitization (123,129) however, the systems involved may vary between cell types (130). These protein kinases are associated with GPCR desensitization that... 

Fig. 3.1. Principal mechanisms of intercellular communiciation. a) communication via extracellular messengers and receptor systems, b) communication via gap junctions. Gap junctions are direct connections between cells. They are coated by proteins (drawn as circles) that can have a regulatory influence on the transport, c) communication via cell surface proteins.

Messenger substances derived from phospholipids can also function as hormones and serve for communication between cells. An important extracellular messenger substance formed from phospholipids is lysophosphatidic acid (l-acyl-sn-glycerine-3-phos-phate). Lysophosphatidic acid (lysophosphatidic acid, LPA) is released by platelets and other cells and reaches its target cells via the circulation. As a product of the blood clotting process, LPA is an abimdant constituent of serum, where it is found in an albu-min-boimd form. 

In 1975, Carswell et al. discovered that this bacterial-induced tumor killing activity could he transferred from one animal to another. When the serum of a mouse that is pretreated with LPS (also called endotoxin) is injected into another animal that has not been treated with endotoxin but harbors a transplantable skin tumor, the tumor undergoes necrosis. In addition, Carswell etal. also showed that addition of this substance (which they dubbed tum<.)r necrosis factor, or TNF) to cultures of transformed cells results in death of the cells (Helson ec al., 1975). It is now known that TNF is a member of a class of proteins called cytokines, which are immune hormones, that is, proteins that are secreted by certain immune cells during stimulation. These proteins are extraordinarily important in the immune response, by acting as the messengers between different cells to relay information (Abbas, 1991). 

Heart muscle fibers are coupled by gap junctions. These intercellular channels provide the exchange of small molecules (<1,000 D), like second messengers, between the cells and they allow electrical coupling. Thus, these cells connected to each other form a syncytium. However, from mapping studies it became evident that under certain conditions, e.g. regional ischemia, the ischemic region uncouples. In addition, mapping studies demonstrated that there is a special activation pattern which accounts for a directed activation of the whole heart. This activation pattern exhibits a considerable similarity from beat to beat. It is well known that the conduction velocity varies between... 

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