Steroid Hormone Secretion.swf / Sekresi Hormon Steroid

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Steroids are lipophilic, low-molecular weight compounds derived from cholesterol that play a number of important physiological roles. The steroid hormones are synthesized mainly by endocrine glands such as the gonads (testis and ovary), the adrenals and (during gestation) by the fetoplacental unit, and are then released into the blood circulation. They act both on peripheral target tissues and the central nervous system (CNS). An important function of the steroid hormones is to coordinate physiological and behavioural responses for specific biological purposes, e.g. reproduction. Thus, gonadal steroids influence the sexual differentiation of the genitalia and of the brain, determine secondary sexual characteristics during development and sexual maturation, contribute to the maintenance of their functional state in adulthood and control or modulate sexual behaviour. It has been recently discovered (review in ref. 1) that, in addition to the endocrine glands, the CNS is also able to form a number of biologically active steroids directly from cholesterol (the so-called "neurosteroids"). These neurosteroids, however, are more likely to have "autocrine" or "paracrine" functions rather than true endocrine effects.

Despite their relatively simple chemical structure, steroids occur in a wide variety of biologically active forms. This variety is not only due to the large range of compounds secreted by steroid-synthesizing tissues, but also to the fact that circulating steroids are extensively metabolised peripherally, notably in the liver, and in their target tissues, where conversion to an active form is sometimes required before they can elicit their biological responses. Steroid metabolism is therefore important not only for the production of these hormones, but also for the regulation of their cellular and physiological actions. This chapter will consider both aspects of steroid metabolism. The emphasis will be on the functional and biological significance of metabolism in endocrine physiology rather than on an extensive description of the metabolic pathways and the enzymes involved.

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