Hormones/hormone secretion steroid

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. 

Adrenocortical hormone (Section 27.6) A steroid hormone secreted by the adrenal glands. There are two types of adrenocortical hormones mineralocorticoids and glucocorticoids. 

Corticosteroids are hormones secreted from the adrenal cortex. These hormones arise from the cortex of the adrenal gland and are made from the crystalline steroid alcohol cholesterol. Synthetic forms of the natural adrenal cortical hormones are available The potent antiinflammatory action of the corticosteroids makes these drugs useful in the treatment of many types of musculoskeletal disorders. The corticosteroids are discussed in Chapter 50. 

Regarding the adrenal steroids, the behavioral activation of hormonal secretion in stress is part of a mechanism for restoring homeostatic balance. For example, an encounter with a predator may require rapid evasive action, in which neural activity and rapidly mobilized hormones such as epinephrine play a role. Adrenal steroid secretion is slower, reaching a peak minutes after the stressful event, and therefore is not expected to play a role in coping with the immediate situation. If the evasive action is successful and... 

So far, the best understood examples of genomic regulation of neuronal function stem from the actions of gonadal and adrenal steroids and thyroid hormone, and many of these actions are involved in the plasticity of behavior that results from hormonal secretion, such as changes in aggressive and reproductive behavior and... 

Matt, D.W., Lee, J., Sarver, P.L., Judd, H.L. and Lu, J.K.H. (1986). Chronological changes in fertility, fecundity, and steroid hormone secretion during consecutive pregnancies in aging rats. Biol. Reprod. 34 478 487. 

Synthesis of steroid hormones Some microorganisms synthesise and secrete steroids that have an immunosuppressive effect. 

Menstrual cycling is dependent upon the hormones secreted by the Graafian follicles. Once the primordial follicles are depleted, the secretion of oestrogen and progesterone progressively fails and menstrual cycle activity gradually decreases, a period which is characterised by irregular cycles. The differences in steroid hormone production or plasma levels between pre- and post-meno-pausal women are as follows ... 

The tenn corticosteroids refers to steroid hormones secreted by the adrenal cortex. Corticosteroids are involved in a wide range of physiologic systems such as stress response, immune response, and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte levels, and behavior. 

Minor changes in the stereochemistry and substitution pattern of the steran skeleton result in vastly different yet specific physiological and pharmacological effects, which in turn influence developmental, metabolic, and behavioral phenomena. The organic chemistry and biochemistry of steroids is the subject of many excellent books and an enormous amount of research and patent literature. This chapter compares and contrasts the structure and mode of action of various steroids, their role in regulating hormonal secretion, and the timing of this regulatory action. 

The synthesis of adrenal steroids and major excreted metabolites is illustrated in Fig. 5.3.1. Little secreted steroid product is excreted unchanged and most of the catabolism takes place in the liver, although cortisol metabolism by the kidney is clinically important and microbial metabolism in the gut can be quantitatively significant. The major metabolic transformations of hormonal steroids and precursors are detailed by Makin [54] and summarized in Fig. 5.3.2. GC-MS steroid profiling is the technique of choice for measurement of important urinary constituents. 

Steroid hormones are secreted on demand from their tissues of origin in response to hormonal signals. The corticosteroids and androgens are made in different regions of the adrenal cortex, and are secreted into blood in response to different signals. 

Pituitary hormone stimulation of steroid hormone synthesis and secretion. 

The occupancy of hormone receptors can fluctuate greatly and is ultimately determined by the concentration of free hormone in the blood. The major determinants of hormone concentrations are (1) the rate of hormone secretion from endocrine cells and (2) the rate of hormone removal by clearance or metabolic inactivation. As we have seen, most hormones (with the exception of steroids) are stored in secretory granules. When the hormone is needed, the granule membranes fuse with the plasma membrane to liberate their contents into the bloodstream. This event is triggered by signals from other hormones or by neural signals. Stimulation of hormonal secretion is usually coupled with an increase of hormone synthesis, so that hormonal stores are replenished. 

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