Principles of Biochemistry

9.1.4.1 Hormones of the Hypothalamus

The hormones secreted by the hypothalamus are relatively short peptides having from 3 to 15 amino acid residues. The hypothalamic hormones do not enter the general circulation but are conveyed directly to the nearby pituitary gland. The pituitary gland has two parts, with different embryological origins, the anterior and posterior lobes.

Anterior pituitary

The anterior pituitary produces several different hormones, all relatively long polypeptides. They are called tropic hormones or tropins, since they have an affinity for the next rank of endocrine glands and stimulate them.

1.Thyrotropic hormone (Thyroid stimulating hormone,TSH): It is a glycoprotein made of 2 polypeptide chain with molecular weight 30,000. It stimulates the synthesis of thyroxine and its release. It binds to specific membrane receptors and activates thyroidal adenylate cyclase.

2. Adrenocorticotropic hormone (ACTH): It is a polypeptide made up of 39 amino acids with molecular weight 4,500. The ACTH stimulates the synthesis of corticosteroids by the adrenal cortex and also enhances their release from the gland.

3. Leutinizing Hormone(LH): In the female, it stimulates final maturation of the graafin follicle, ovulation, and the development of the corpora lutea. The secretion of estrogen and progestrone are also stimulated. In the male it stimulates testosterone production by the testis which maintains spermatogenesis and causes the development of accessory sex organs such as the vas deferens, prostrate and seminal vesicles. It increases cAMP.

4. Follicle Stimulating Hormone (FSH): In the female, it stimulates the growth and maturation of graafin follicle and prepare them for ovulation and for the action of LH and enhances the release of estrogens which is very active during menstrual cycle. In the male it stimulates seminal tubule and also the early stages of spermatogenesis.

5.Growth hormone(GH, Somatotrophin): It increases total growth and causes gigantism in children. It stimulates production of somatomedins from liver and kidney which has insulin-like activity.

6. Prolactin(PL): It is a protein with a molecular weight 23,000.It activates corpus luteum and stimulates the production of progesterone by the developed corpus luteum. It increases during pregnancy and stimulates mammary development and growth hormone like metabolic changes.

Posterior pituitary

The posterior pituitary releases two characteristic hormones, oxytocin and vasopressin.

1.oxytocin: It is a cyclic poly peptide containing 8 amino acids with molecular weight of 1000. It causes contraction of smooth muscles in the mammary gland resulting in milk excretions. It stimulates the contraction of gall bladder, intestine and urinary bladder. It causes uterine contraction during labor.

2.Vasopressin: It is a cyclic polypeptide containing 8 amino acids. It increases blood pressure and increases water reabsorption in the kidney. It is also called as antidiuretic hormone.