Hormones tropic

Nerve impulses stimulate the hypothalamus to send specific hormones to the pituitary gland, thus stimulating (or inhibiting) the release of tropic hormones. The anterior pituitary hormones in turn stimulate other endocrine glands (thyroid, adrenals, pancreas) to secrete their characteristic hormones, which in turn stimulate specific target tissues. 

The adenohypophysis is the part of the gland in which the tropic hormones are secreted They include the adrenocorticotropic hormone lACTHl. die thyrotropic hormone (TSH). and. somatotropin, as well as three hormones with pronounced effects upon the gonads the hormone prolactin, the follicle-stimulating hormone tFSHl and the luteinizing or interstitial cell stimulating hormone tLH or ISCH)... 

Wl. Wada, H. G., Danisch, R. J., Baxter, S. R., Federici, M. M., Fraser, R. C., Brownmill-er, L. J., and Lankford, J. C., Enzyme immunoassay of the glycoprotein tropic hormones—choriogonadotropin, lutropin, thyrotropin—with solid-phase monoclonal antibody for the a-subunit and enzyme-coupled monoclonal antibody specific for the fi-subunit. Clin. Chem. 28, 1862-1866 (1982). 

Figure 16.1 Hypothalamic-pituitary system. The hypothalamus receives various types of impulses and responds by secreting appropriate release and release-inhibiting factors. These migrate to the anterior or intermediate pituitary via the hypophyseal portal vein system and elicit the secretion of various tropic or non tropic hormones. For instance, when the organism is exposed to cold, blood TSH levels increase when under stress, blood ACTH levels rise. In some animals, the absence of light causes the release of a-MSH.

Regulation of synthesis Thyroid function is controlled by a tropic hormone, thyroid-stimulating hormone (TSH, thyrotropin), a glycoprotein synthesized by the anterior pituitary (see Figure 25.2). TSH generation is governed by the hypothalamic thyrotropin-... 

Fail PA, Anderson S A, Friedman MA (1999) Response of the pituitary and thyroid to tropic hormones in Sprague-Dawley versus Fischer 344 male rats. Toxicol Sci 52 107-121 Kyung OY, Narayanan L, Mattie DR, Godfrey RJ, Todd PN, Sterner TR, Mahle DA, Lumpkin MH, Fisher JW (2002) The Pharmacokinetics of Perchlorate and Its Effect on the Hypothalamus-Pituitary-Thyroid Axis in the Male Rat. Toxicol Applied Pharmacol 182 148-159... 

Hormones synthesised by the antei ior pituitary that regulate the activity of various endocrine glands are referred to as tropic hormones (e.g. thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), luteinizing hormone (LH), follicle-stimulating hormone (FSH)). 

The hypotlialamic peptides, in contrast to some pituitary tropic hormones such as the gonadotropins, are not entirely species specific. Porcine Gn-RH and TRH have been shown to have an effect in humans. The availability of highly purified and synthetic releasing hormones has important diagnostic and therapeutic implications. CRH, GH-RH, Gn-RH, and TRH have all been used to test for pituitary hormone... 

Eddy RL, Jones AL, Chakmakjian ZH, Sdverthorne MC. Effects of levodopa (L-dopa) on human hypophyseal tropic hormone release, J Clin Endocrinol Metab 1971 33 709-12. 

It goes without saying that whatever methods of measurements are used, stimulation and suppression tests using ACTH, prednisone, or dex-amethasone, and in some cases chorionic gonadrotropin, other pituitary tropic hormones and steroid sex hormones, are an essential part of any chemical diagnostic procedures in this field. 

Animals employ several mechanisms to prevent excessive hormone synthesis and release. The most prominent of these is feedback inhibition. The hypothalamus and anterior pituitary are controlled by the target cells they regulate. For example, TSH release by the anterior pituitary is inhibited when blood levels of T3 and T4 rise (Figure 16.10). The thyroid hormones inhibit the responsiveness of TSH-synthesizing cells to TRH. In addition, several tropic hormones inhibit the synthesis of their releasing factors. 

Progesterone is synthesized in the ovaries, the adrenal glands, and the placenta. In a nonpregnant woman, it is produced by the corpus luteum during the latter part of the menstrual cycle under the influence of luteinizing and luteo-tropic hormones. In a pregnant woman, it is produced initially by the corpus luteum under the influence of chorionic gonadotropins, and is synthesized by the placenta after failure of the corpus luteum. 

Most pituitary hormones act on endocrine glands that occupy an intermediate, or secondary, position in the hierarchy, stimulating them to produce hormones that exert the ultimate actions on target tissues. Pituitary hormones that act on other endocrine glands are called tropic hormones or tropins. An example is adrenal corticotropic hormone (ACTH), also called / -corticotropin (see here also). This peptide is secreted from the anterior pituitary, and it stimulates the adrenal cortex to produce glucocorticoids and mineralocorticoids, which in turn act on a number of tissues. 

FIGURE 2.1 Representation of the forward pathways of pituitary and target gland hormone release and action -------tropic hormones -----------tissue-affecting hormones. 

The translocator protein (TSPO) also participates in cholesterol mobilization, particularly with pharmacologic stimulation [23], but knockout of mouse TSPO shows it is not required for tropic hormone stimulation of steroidogenesis [24a]. 

Tropic hormones, including adrenocorticotro-pin (ACTH) and angiotensin II in the adrenal and luteinizing hormone (LH) in the ovary and testis, signal via second messengers to drive acute rises in steroid production from matty steroidogenic cells. StAR mediates this acute rise in steroido-... 

The actions of hormones are very diverse. Some, e.g. glucagon, have specific and clearly defined effects on the metabolism of particular tissues. Others such as growth hormone and the sex hormones have more general effects on growth and development while the tropic hormones stimulate their target tissues to secrete further hormones. For example the adrenocorticotropic hormone (ACTH), which is produced in the anterior pituitary, promotes the synthesis and release of glucocorticoids by the adrenal cortex. A list of some of the hormones and their effects on metabolism is given in Table 24.1. 

Our approach to this subject was to develop an to vitro model system in which we could study the effects of tropic hormones upon steroidogenesis to the corpus luteum under precisely controlled conditions. Essentially the system we worked out was to prepare slices of a single corpus luteum, obtained from a pregnant cow at slaughter, and to incubate these slices in Krebs-Ringer bicarbonate buffer in the presence or absence of test substances (3). After the incubation (usually 2 hours) progesterone was isolated from the tissue and medium, and measured spectrophotometrically. 

That the nature of the adrenal secretion may be considerably altered in a single species by ACTH administration has been observed in the rabbit by Kass et al. (1954). These workers observed that prolonged administration of the tropic hormone increased the amount of cortisol and decreased the amount of corticosterone secreted. This observation was essentially confirmed by Krum and Glenn (1965), and the mechanism was further studied by Fevold (1967, 1969). 

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