Hormones control mechanisms

The sexual dimorphism of cuticular hydrocarbons is completed during the first three days after imaginal eclosion. During the same period, important physiological events take place, female oocyte maturation and vitellogenesis. A number of mutations have been described which affect ovarian development or endocrine control. These mutants were used to elucidate a possible hormone control mechanism used to regulate hydrocarbon biosynthesis. 

Figure 3.2. Schematic illustration of the hormonal control mechanisms of the prostate cell.

The various hormone antagonists not only help in giving a vivid picture of hormonal control mechanisms but also serve as immensely viable tool for the management and control of hormone-dependent cancer. This has, in fact, generated enough interest towards the development of newer and altogether safer hormone antagonists. 

Tramontano, D., Rotella, CM., Toccafondi, R., Ambesi-Impiombato, S.F., 1986c, Thyrotropin-independent mutant clones from FRTL-5 rat th3 oid cells hormonal control mechanisms in differentiated cells. Endocrinology 118 862. 

Kimball, R. T. 2006. Hormonal control of coloration. In Bird Coloration, Volume I, Mechanisms and Measurements (Hill, G. E. and McGraw, K. J., eds.). Harvard University Press, Cambridge, MA, pp. 431-468. 

In the uterus the SR is a pronounced and complex organelle, which appears to be under some degree of hormonal control. For example, its size, SERCA expression and release mechanisms show alteration with pregnancy. As yet there has been little study of the molecular and mechanistic processes that lead to these changes in the... 

The mechanisms of all these alterations in G-6-PDH and 6-PGDH activities are as yet unknown and will have to be clarified. A special question is whether or not we can consider them as real hormonal control or as a simple coincidence conditioning alterations in equilibria of enzyme reactions involved. 

The treatment of animals with growth-promoting hormones is a common practice in conventional agriculture outside of the EU. The effects of this practice are still not predictable in an entirely reliable way with respect to the toxic and carcinogenic effects of their residuals on humans (Collins et al. 1989). Although their use was banned in the EU several years ago, satisfactory controlling mechanisms have not been established. 

In all organisms, carbohydrate metabolism is subject to complex regulatory mechanisms involving hormones, metabolites, and coenzymes. The scheme shown here (still a simplified one) applies to the liver, which has central functions in carbohydrate metabolism (see p. 306). Some of the control mechanisms shown here are not effective in other tissues. 

Several other mechanisms also regulate cholesterol synthesis (Fig. 21-44). Hormonal control is mediated... 

Synthesis of many enzymes is repressed most of the time. The appearance of an enzyme at a particular stage in the life of an organism as well as the differing distributions of isoenzymes within differentiated tissue result from derepression. The control of enzyme synthesis may also be exerted during the splicing of transcripts and at the translational level as well. These control mechanisms are often relatively slow, with response times of hours or even days. However, effects on the synthesis of some hormones, such as insulin (Section G), may be observed within a few minutes. 

One of the insect neurohormones, the activation hormone, controls the secretion of the corpora allata, paired glands that synthesize the juvenile hormone (Fig. 22-4) in insect larvae. While the structure of the juvenile hormone varies somewhat with species, it is usually a polyprenyl ester. A specific binding protein provides the hormone with protection from degrada-tive enzymes. However, in the tobacco homworm an esterase, able to hydrolyze the protein-bound juvenile hormone, is produced at the start of pupal differentiation.354 The exact mechanism of action of juvenile hormones has been difficult to determine. However, it affects polyamine synthesis.355 356... 

Use of Hormones to Alter Normal Endocrine Function. Because of the intrinsic control mechanisms in the endocrine system, administration of exogenous hormones can often affect the normal release of hormones. This fact can be exploited in certain situations to cause a desired change in normal endocrine function. For instance, oral contraceptives containing estrogen and progesterone inhibit ovulation by inhibiting the release of LH and FSH from the anterior pituitary. 

Increased clearance of steroid hormones due to induction of hepatic biotransformation enzymes following chemical exposure often has been cited as a possible mechanism by which toxicants could lower circulating testosterone or 17/3-estradiol levels. While enhanced clearance of sex steroids has been demonstrated following chemical exposure and induction of hepatic biotransformation enzymes, elegant feedback control mechanisms tend to ensure that more hormone is produced and homeostasis is maintained (Figure 17.2). Enhanced clearance of sex steroids can contribute to endocrine disruption if the toxicity also results in impaired hormone synthesis (i.e., gonadal toxicity or interference with the feedback control of hormone synthesis). 2,3,7,8-Tetrachlorodibenzodioxin appears to lower circulating sex steroid levels via this dual effect. 

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process. 

Bamberg, E. Schoner, W. (eds.) (1994). The Sodium Pump. Structure, Mechanism, Hormonal Control and its Role in Disease. Steinkopff, Darmstadt. 

This phenomenon, which varies in intensity in different varieties, is characteristic of many other perennial fruit crops in which flower initials are laid down in the summer of the year before flowering. The sparse flower induction which accompanies a heavy crop was formerly attributed to depletion of the carbohydrate and nitrogenous reserves of the tree but, whilst alternate bearing may have developed as a means by which the tree could conserve its food reserves, the control mechanisms are clearly hormonal in nature. 

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