Anterior pituitary (adenohypophysis), the master. mas endocrine gland, elaborates number of trot important regulatory hormones. All of these are Ihd peptide in nature and act at extracellular recep- GH tors located on their target cells. Their secretion is tiss controlled by the hypothalamus through releasing Th and release-Inliibitoru hormones that are trans- ind ported via hypothalamohypophyseal portal th system, and is subjected to feedback inhibitionby za hormones of their target glands. Each anterior gl pituitary hormone is produced by a separate r group of cells, which according to their staining characteristic are either acidophilic or basophilic. ( The acidophils are either somatotropes-GH; or lactotropes – Prolactin. The basophils are gonadotropes- FSH and LH; thyrotropes – TSH; and corticotrope-lipo- tropes- ACTH. The latter in addition to ACTH also produce two melanocyte stimulating hormones (MSHs) and two lipotropins, but these are probably not important in man.
It is a 191 amino acid, single chain peptide of MIW 22000.
GH promotes growth of all organs by inducing hyperplasia. In general, there is a proportionate increase in the size and mass of all parts, but in the absence of gonado- tropins, sexual maturation does not take place. The growth of brain and eye is independent of GH. It promotes retention of nitrogen and other tissue constituents: more protoplasm is formed. The positive nitrogen balance results from increased uptake of amino acids by tissues and their svnthesis into proteins. GH promotes utili- zation of fat and spares carbohydrates: uptake of glucose by muscles is reduced while its output from liver is enhanced; fat is broken down.
The growth promoting, nitrogen retaining and certain metabolic actions of GH are exerted indirectly through the elaboration of peptides called Sontatomedins or Insulin-like grotth factors (mainly IGF-1, also IGF-2) which are extracellular mediators of GH response. Liver is the major. source of circulating IGF-1, while IGF-1 produced by other target cells acts locally in a paracrine manner. Like insulin, IGF-1 promotes lipogenesis and glucose uptake by muscles. The IGF-1 receptor also is structurally and functionally | analogous to the insulin receptor.
The anterior pituitary secretes two Gns tiz. FSH and LH. Both are glycoproteins containing 23- 28% sugar and consist of two peptide chains having a total of 207 amino acid residues. FSH has MW 32,000 while LH has MW 30,000.
FSH and LH act in concert to promote gametogenesis and secretion of gonadal hormones. FSH In the female it induces follicular growth, development of ovum and secretion of estrogens. In the male it supports spermatogenesis and has a trophic influence on seminiferous tubules. Ovarian and testicular atrophy occurs in the absence of FSH. LH It induces preovulatory swelling of the ripe graafian follicle and triggers ovulation followed by luteinization of the ruptured follicle and sustains corpus luteum till the next menstrual cvcle. It is also probably responsible for atresia of the remaining follicles. Progesterone secretion occurs only under the influence of LH. In the male. LH stimulates testosterone secretion by the interstitial cells and is designated inter-stitial cll stimulating hormone (ICSH).