lipids in the membrane. A series of synthetic reactions catalyzed by enzymes take place in the membrane, culminating in the release of the PG product from the membrane into the cellular cytoplasm. The released PG may bind to its receptor located within the plasma membrane or other internal membrane, or it may be released to the cell exterior and ultimately produce an effect by binding to a receptor in the cell membrane of a neighboring cell. There is little information available on the mechanism by which PGs are secreted from cells.

PGs are produced by many different cells in the body. It is not yet clear whether all cells are capable of producing them, but this seems a distinct possibility. Ultimately, the release of PGs from cells may be a product of the action of other hormones or neurotransmitters (i.e., the signal to the cell to synthesize PGs).

PGs exert a wide variety of effects on different target tissues. They affect behavior through direct actions on individual neurons and substructures of the brain, such as cerebellar and reticular formation (the latter is responsible for screening various kinds of environmental signals), and they act on the hypothalamus and the pituitary. Vasomotor and temperature-regulatory centers are affected by PGs. Autonomic and neuromuscular junctions are also affected. As is evident in other chapters, PGs act on anterior pituitary trophic hormone target tissues such as thyroid, adrenal, ovary, and testis, on exocrine hormone targets such as pancreas and gastric mucosa, and on endocrine target tissues such as renal tubules, bone, and adipocytes. They act on the smooth muscles of the reproductive, alimentary, and respiratory tracts and on cardiovascular smooth muscles. Some of these effects will be elaborated in this chapter. PGs act on red blood cells, leukocytes, and platelets, the last of which will be described here. The role of PGs in pain will be mentioned, consistent with the inflammatory actions of certain PGs (see Chapter 10). The roles of prostaglandins and prostaglandin relatives in asthma will be discussed.

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