Transport of hormones

As discussed in the previous section, steroid and thyroid hormones are transported in the blood bound to plasma proteins. The serum concentrations of free hormone (H), plasma protein (P), and bound hormone (HP) are in equilibrium  

When the concentration of the free form of a hormone decreases, then more of this hormone will be released from the binding proteins. The free hormone is the biologically active form. It binds to the target tissue to cause its actions and is involved with the negative feedback control of its secretion. The binding of hormones to plasma proteins has several beneficial effects, including  

Steroid and thyroid hormones are minimally soluble in the blood. Binding to plasma proteins renders them water soluble and facilitates their transport. Protein binding also prolongs the circulating half-life of these hormones. Because they are lipid soluble, they cross cell membranes easily. As the blood flows through the kidney, these hormones would enter cells or be 

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Blood serum, usually bovine-derived (calf or fetal bovine), contains amino acids, growth factors, vitamins, proteins, hormones, lipids, and minerals, among other components, as indicated in Table 5.3. Besides fetal bovine serum, serum from horse (equine), and even from humans (less common) can also be used. The main functions of serum are to stimulate growth and other cellular activities through hormones and growth factors, to increase cellular adhesion through specific proteins, and to supply proteins for the transport of hormones, minerals, and lipids (Freshney, 2005). Supplementation with bovine fetal serum is performed at concentrations from 2 to 20% in volume. 

Most lipid-soluble hormones in the blood are bound to specialized carrier proteins. The availability of hormones for physiological functions depends on the total concentration of the hormone as well as the amount of hormone existing in the free state protein-bound hormones are not readily available for receptor binding. While lack of carrier proteins could impair the transport of hormones to target organs, excessive amounts may decrease the availability of free hormones. 

Blood is also the medium of transport of hormonal signals from one tissue to another, and of exit for metabolic end products, such as urea, via the kidneys. 

Receptors located on the plasma membrane, in the cytoplasm, and in mitochondria have been described as well (36). Plasma membrane receptors are believed to mediate the transport of hormone into the cell and, possibly, to mediate nonnuclear, immediate effects of thyroid hormone (36). The presence of a large number of low-affinity cytosol binding proteins has been known for ... 

The existence of a closed circulatory system in higher animals provides the organism with an easy and efficient route for the transport of hormones from the site of synthesis to the target tissues. In plants some hormones appear to be transported directly in the vascular tissue for example, cytokinin, GA, and ABA move from the root to the shoot in the xylem GA moves out of young leaves in the phloem and ABA is transported out of wilting leaves in the phloem (Fig. 6.1). However, auxin is not transported directly in the vascular tissue, but instead appears to be transported in cells associated with the phloem (Fig. 6.1). Ethylene poses a special problem in that it is a diffusable gas. However, its precursor, 1-aminocyclopropane-l-carboxylic acid (ACC), is transported from the root to the shoot in the xylem. Therefore, using the traditional concept of a hormone as a translocated chemical messenger, ACC may be more aptly considered to be a hormone than ethylene. 

Transport in the blood is no longer a requisite for a hormonal response. Responses can occur after release of hormones into the interstitial fluid with binding to receptors in nearby ceUs, called paracrine control, or binding to receptors on the ceU that released the hormone, called autocrine control. A class of hormones shown to be synthesized by the tissue in which they act or to act in the local ceUular environment are the prostaglandins (qv). These ubiquitous compounds are derived from arachidonic acid [506-32-1] which is stored in the ceU membranes as part of phosphoHpids. Prostaglandins bind to specific ceUular receptors and act as important modulators of ceU activity in many tissues. 

Three hormones regulate turnover of calcium in the body (22). 1,25-Dihydroxycholecalciferol is a steroid derivative made by the combined action of the skin, Hver, and kidneys, or furnished by dietary factors with vitamin D activity. The apparent action of this compound is to promote the transcription of genes for proteins that faciUtate transport of calcium and phosphate ions through the plasma membrane. Parathormone (PTH) is a polypeptide hormone secreted by the parathyroid gland, in response to a fall in extracellular Ca(Il). It acts on bones and kidneys in concert with 1,25-dihydroxycholecalciferol to stimulate resorption of bone and reabsorption of calcium from the glomerular filtrate. Calcitonin, the third hormone, is a polypeptide secreted by the thyroid gland in response to a rise in blood Ca(Il) concentration. Its production leads to an increase in bone deposition, increased loss of calcium and phosphate in the urine, and inhibition of the synthesis of 1,25-dihydroxycholecalciferol. 

The influx of Ca(Il) across the presynaptic membrane is essential for nerve signal transmission involving excitation by acetylcholine (26). Calcium is important in transducing regulatory signals across many membranes and is an important secondary messenger hormone. The increase in intracellular Ca(Il) levels can result from either active transport of Ca(Il) across the membrane via an import channel or by release of Ca(Il) from reticulum stores within the cell. More than 30 different proteins have been linked to regulation by the calcium complex with calmoduhn (27,28). 

Fig. 2. Schematic representation of relevant electrolyte transport through the renal tubule, depicting the osmolar gradient ia medullary iaterstitial fluid ia ywOj yW where represents active transport, —passive transport, hoth active and passive transport, and passive transport of H2O ia the presence of ADH, ia A, the cortex, and B, the medulla. An osmole equals a mole of solute divided by the number of ions formed per molecule of the solute. Thus one mole of sodium chloride is equivalent to two osmoles, ie, lAfNaCl = 2 Osm NaCl. ADH = antidiuretic hormone.

Effects on Synthesis, Storage, Release, Transport and Clearance of Hormones... 

Membrane asymmetries in the transverse direction (from one side of the membrane to the other) can be anticipated when one considers that many properties of a membrane depend upon its two-sided nature. Properties that are a consequence of membrane sidedness include membrane transport, which is driven in one direction only, the effects of hormones at the outsides of cells, and the immunological reactions that occur between cells (necessarily involving only the outside surfaces of the cells). One would surmise that the proteins involved in these and other interactions must be arranged asymmetrically in the membrane. 

We turn now to the biosynthesis of lipid structures. We begin with a discussion of the biosynthesis of fatty acids, stressing the basic pathways, additional means of elongation, mechanisms for the introduction of double bonds, and regulation of fatty acid synthesis. Sections then follow on the biosynthesis of glyc-erophospholipids, sphingolipids, eicosanoids, and cholesterol. The transport of lipids through the body in lipoprotein complexes is described, and the chapter closes with discussions of the biosynthesis of bile salts and steroid hormones. 

The CaR regulates numerous biological processes, including the expression of various genes (e.g., PTH) the secretion of hormones (PTH and calcitonin), cytokines (MCP-1), and calcium (e.g., into breast milk) the activities of channels (potassium channels) and transporters (aquaporin-2) cellular shape, motility (of macrophages), and migration cellular adhesion (of hematopoietic stem cells) and cellular proliferation (of colonocytes), differentiation (of keratinocytes), and apoptosis (of H-500 ley dig cancer cells) [3]. 

H)2-D3 is produced by a complex series of enzymatic reactions that involve the plasma transport of precursor molecules to a number of different tissues (Figure 42-9). One of these precursors is vitamin D—really not a vitamin, but this common name persists. The active molecule, l,25(OH)2-D3, is transported to other organs where it activates biologic processes in a manner similar to that employed by the steroid hormones. 

The hydrophihc hormones—generally class II and of peptide stmcture—are freely soluble in plasma and do not require transport proteins. Hormones such as insulin, growth hormone, ACTH, and TSH circulate in the free, active form and have very short plasma half-... 

Another important function of albumin is its ability to bind various ligands. These include free fatty acids (FFA), calcium, certain steroid hormones, bilirubin, and some of the plasma tryptophan. In addition, albumin appears to play an important role in transport of copper in the human body (see below). A vatiety of drugs, including sulfonamides, penicilhn G, dicumarol, and aspirin, are bound to albumin this finding has important pharmacologic implications. 

NIELSEN K K, BUDDINGTON K K RAUN, K, HANSEN T K and BUDDINGTON R K. (2002) AbsOrption and systemic availability or two synthetic growth hormone secretogogues and transport of glucose by the proximal small intestine of anestrus dogs after administering estradiol. J Comp Physiol. In press. 

Although suppression of FSH and LH is the primary mechanism by which combined oral contraceptives prevent ovulation, there are other mechanisms by which these hormones work to prevent pregnancy. Other mechanisms include reduced penetration of the egg by sperm, reduced implantation of fertilized eggs, thickening of cervical mucus to prevent sperm penetration into the upper genital tract, and slowed tubal motility, which may delay transport of sperm.1 Thus, in... 

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