Hormonal controls interactions

Compartmental models are a class of dynamic, that is, differential equation, models derived from mass balance considerations, which are widely used for quantitatively studying the kinetics of materials in physiologic systems. Materials can be either exogenous, such as a drug or a tracer, or endogenous, such as a substrate or a hormone, and kinetics include processes such as production, distribution, transport, utilization, and substrate-hormone control interactions. 

The lactogenic hormone of the pituitary is one that has been known for a relatively long time and has been purified. The hormonal control of lactation and of the development of the mammary glands is complicated by several endocrine interactions, but the lactogenic hormone is a potent and highly important agent. 

Fig. 12. A pictorial representation of adenylate cyclase and hormone receptors interacting with a membrane. The GTP control component is also shown. The different hormone receptors may not interact directly with the adenylate cyclase and may be diffusing freely in the membrane until a hormone is bound.

A number of other, more specialized systems are available to the reproductive toxicologist to answer specific mechanistic questions. In particular, the hormonal control of reproductive function and its perturbation by toxicants have received much attention. Such investigations can use intact cells to investigate the downstream consequences of toxicants on hormone-receptor interactions or use cells that respond to specific hormones (e.g., the MCF-7 breast cancer cell line and estrogen Soto et al., 1995). Receptor biology/ligand binding can be examined in membrane preparations of specific cell types or in recombinant cell systems (e.g., human and yeast cells Klein et al.,... 

The thyroid and parathyroid glands serve a number of vital endocrine functions. The thyroid gland synthesizes and secretes the thyroid hormones T3 and T4. These hormones are important regulators of cellular metabolism and metabolic rate. Thyroid hormones also interact with other hormones to facilitate normal growth and development. The parathyroid glands control calcium homeostasis through the release of PTH. This hormone is crucial... 

An important interaction between insulin and glucagon may also take place directly within the pancreas, and insulin appears to be the dominant hormone controlling this interaction.29,53 When the beta cells sense an increase in blood glucose, they release insulin, which in turn inhibits glucagon release from the alpha... 

Figure 16.11 Control of catecholamine biosynthesis in the adrenal medulla. TH, tyrosine hydroxylase DBH, dopamine hydroxylase PNMT, phenylethanolamine methyl-transferase ACTH, adrenocorticotropic hormone. The heavy arrows indicate major sites of regulation. (Reproduced by permission from Axelrod, J. Reisine TD. Stress hormones their interaction and regulation. Science 224 452-459, 1984.)...

Hull EM, Lorrain DS, Du J, Matuszewich L, Lumley LA, Putnam SK, Moses J (1999) Hormone-neurotransmitter interactions in the control of sexual behavior. Behav Brain Res 705 105-116. 

Retinoids (derivatives of retinol) act like steroid hormones and interact with specific receptor proteins in the cell nucleus. The ligand-receptor complexes bind to specific DNA sequences, where they control the transcription of particular genes. 

Insulin and the counterregulatory hormones exert two types of metabolic regulation (see Chapter 26). The first type of control occurs within minutes to hours of the hormone-receptor interaction and usually results from changes in the catalytic activity or kinetics of key preexisting enzymes, caused by phosphorylation or dephosphorylation of these enzymes. The second type of control involves regulation of the synthesis of key enzymes by mechanisms that stimulate or inhibit transcription and translation of mRNA. These processes are slow and require hours to days. 

Interactions involved in normal gonadal function and hormonal communication are very complex. Any of these loci may be involved mechanistically in a toxicant s endocrine-related effect Such impaired hormonal control may occur as a consequence of altered hormone synthesis, storage/release, transport/clearance, receptor recogniiion/binding, or postre-ceptor responses. 

FIGURE 2.3 The interaction of insulin as an illustration of negative feedback within a hormonal control system. 

Demski, L. S., and Hornby, P. J., 1982, Hormonal control of fish reproductive behavior brain-gonadal steroid interactions. Can. J. Fish. Aquatic Sci., 39 36. 

There are a number of possibilities that could account for the interaction between thyroid hormone and NGF. For example, treatments with insulin and insulin-like growth factor II have been found to increase specific and saturable NGF binding sites in cultured human neuroblastoma cells.It is possible that analogous mechanisms between thyroid hormone and NGF may operate in subcortical cholinergic cells. Another possibility could be that if thyroid hormone controls the formation of ChAT or of proteins essential for cholinergic cell maturation at a pretranslational level S and NGF regulates the de novo synthesis of these crucial proteins at transcriptional level,30 then a combination of these effects could synergistically potentiate ChAT activity. 

Another important factor of hormone-hormone interaction is the hormonal control of hormonal movement and polarity. Thus, various cytokinins have been reported to increase the polar movement of indole acetic acid and vice versa. Gibberellin treatment was shown also under cer- bain circumstances to increase the basipetal auxin movement. On the other hand, abscisic acid decreases the auxin movement and ethylene, according to some evidence, the gibberellin movement, perhaps through a promotion of conjugation of the auxins with aspartate and of the gibberellins with glucose (cf. Eef. 27). 

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