Topic 1: CLASSIFICATION OF HORMONES AND PLACENTAL HORMONES B.Sc. Part-II

Paper 4 PHYSIOLOGY: CONTROLLING AND COORDINATING SYSTEMS

CLASSIFICATION OF HORMONES Chemically, hormones are divided into two broad classes and each class differs in its biosynthetic pathway Lipid-soluble Hormones The lipid-soluble hormones are steroid hormones, thyroid hormones, and nitric oxide. 1. Steroid hormones are derived from cholesterol. Each steroid hormone is unique due to the presence of different

chemical groups attached at various sites on the four rings at the core of its structure. These small differences allow for a large diversity of functions.

2. Two thyroid hormones (T3 and T4) are synthesized by attaching iodine to the amino acid tyrosine. The benzene ring of tyrosine plus the attached iodines make T3 and T4 very lipid soluble. 3. The gas nitric oxide (NO) is both a hormone and a neurotransmitter. Its synthesis is catalyzed by the enzyme nitric oxide synthase.

Water-soluble Hormones The water-soluble hormones include amine hormones, peptide and protein hormones, and eicosanoid hormones 1. Amine hormones are synthesized by decarboxylating (removing a molecule of CO2) or modifying certain amino acids.

They are called amines because they retain an amino group. • The catecholamines- epinephrine, norepinephrine, and dopamine are synthesized by modifying the amino acid

tyrosine. • Histamine is synthesized from the amino acid histidine by mast cells and platelets • Serotonin and melatonin are derived from tryptophan 2. Peptide hormones and protein hormones are amino acid polymers • The smaller peptide hormones consist of chains of 3 to 49 amino acids • The larger protein hormones include 50 to 200 amino acids Antidiuretic hormone and oxytocin- peptide hormone human growth hormone and insulin- protein hormone thyroid-stimulating hormone (have attached carbohydrate groups)- glycoprotein hormones 3. The eicosanoid hormones are derived from arachidonic acid, a 20-carbon fatty acid • The two major types of eicosanoids are prostaglandins and leukotrienes • The eicosanoids are important local hormones, and they may act as circulating hormones as well

Endocrine glands and the hormones secreted by each gland

REGULATION OF HORMONE SECRETION

• The release of most hormones occurs in short bursts, with little or no secretion between bursts. When stimulated, an endocrine gland will release its hormone in more frequent bursts, increasing the concentration of the hormone in the blood.

• In the absence of stimulation, the blood level of the hormone decreases. Regulation of secretion normally prevents

overproduction or underproduction of any given hormone. Hormone secretion is regulated by (1) signals from the nervous system (2) chemical changes in the blood (3) Effect of other hormones • Nerve impulses to the adrenal medullae regulate the release of epinephrine • Blood Ca2 level regulates the secretion of parathyroid hormone • Adrenocorticotropic hormone stimulates the release of cortisol by the adrenal cortex Most hormonal regulatory systems work via negative feedback but a few operate via positive feedback Example- during childbirth, the hormone oxytocin stimulates contractions of the uterus, and uterine contractions in turn stimulate more oxytocin release, a positive feedback effect.

Action of Lipid-soluble Hormones • lipid-soluble hormones, including steroid hormones and thyroid

hormones, bind to receptors within target cells. Their mechanism of action is as follows: 1. Lipid-soluble hormone molecule diffuses from the blood, through

interstitial fluid, and through the lipid bilayer of the plasma membrane into a cell.

2. The hormone binds to and activates receptors located within the cytosol or nucleus. The activated receptor–hormone complex then alters gene expression: It turns specific genes of the nuclear DNA on or off

3. DNA is transcribed, new messenger RNA (mRNA) forms, leaves the nucleus, and enters the cytosol. synthesis of a new protein takes place

4. The new proteins alter the cell’s activity and cause the responses of that hormone.

MECHANISMS OF HORMONE ACTION

Action of Water-soluble Hormones

A water-soluble hormone (the first messenger) diffuses from the blood through interstitial fluid and then binds to its receptor at

the exterior surface of a target cell’s plasma membrane. The hormone–receptor complex activates a membrane protein called a

G protein. The activated G protein in turn activates adenylate cyclase.

Adenylate cyclase converts ATP into cyclic AMP (cAMP)

cAMP (the second messenger) activates protein kinases which phosphorylates cellular proteins and ATP converted to ADP

Phosphorylation activates some of these proteins and inactivates others, rather like turning a switch on or off.

Phosphorylated proteins cause physiological responses

Phosphodiesterase inactivates cAMP. Cell’s response is turned off unless new hormone molecules continue to bind to their

receptors in the plasma membrane

PLACENTAL HORMONES

• Human chorionic -stimulates the corpus luteum in the ovary gonadotropin (hCG) to continue the production of estrogens and progesterone to maintain pregnancy • Estrogens- maintain pregnancy and help prepare and progesterone mammary glands to secrete milk. • estrogens, progesterone, and relaxin - secreted by corpus luteum of pregnancy • Relaxin -helps maintain pregnancy by inhibiting myometrial contractions

• The placenta produces sufficient estrogen and progesterone from maternal and fetal precursors to take over the function

of the corpus luteum after the sixth week of pregnancy. • Ovariectomy before the sixth week leads to abortion, but ovariectomy there after has no effect on the pregnancy. The

function of the corpus luteum begins to decline after 8 wk of pregnancy, but it persists throughout pregnancy. hCG secretion decreases after an initial marked rise, but estrogen and progesterone secretion increase until just before parturition.

Human Chorionic Gonadotropin • hCG is a glycoprotein that contains galactose and hexosamine produced by the syncytiotrophoblast

• Like the pituitary glycoprotein hormones, it is made up of α and β subunits

• hCG-α is identical to the α subunit of LH, FSH, and TSH

• hCG is primarily luteinizing and luteotropic and has little FSH activity

• It can be measured by radioimmunoassay and detected in the blood as early as 6 d after conception. Its presence in

the urine in early pregnancy is the basis of the various laboratory tests for pregnancy and it can be detected in the urine as early as 14 d after conception

Rates of secretion of estrogens and progesterone, and concentration of human chorionic gonadotropin at different stages of pregnancy.

Human Chorionic Somatomammotropin/chorionic growth hormone-prolactin (CGP)/ human placental lactogen (hPL)/ human chorionic somatomammotropin (hCS) • Protein secreted by the placenta at about the fifth week of pregnancy • Secretion of this hormone increases progressively throughout the remainder of pregnancy in direct proportion to the

weight of the placenta • The syncytiotrophoblast also secretes large amounts of a protein hormone that is lactogenic and has a small amount of

growth-stimulating activity • Human chorionic somatomammotropin causes decreased insulin sensitivity and decreased utilization of glucose in the

mother, thereby making larger quantities of glucose available to the fetus

• The hormone promotes the release of free fatty acids from the fat stores of the mother, thus providing this alternative source of energy for the mother’s metabolism during pregnancy

• Functions as a “maternal growth hormone of pregnancy” to bring about the nitrogen, potassium, and calcium retention, lipolysis, and decreased glucose utilization

Secretion of Relaxin by the Placenta • Relaxin, is secreted by the corpus luteum of the ovary and by placental tissues • Its secretion is increased by a stimulating effect of human chorionic gonadotropin at the same time that the corpus

luteum and the placenta secrete large quantities of estrogens and progesterone

• It has also been claimed that relaxin softens the cervix of the pregnant woman at the time of delivery

Secretion of Progesterone by the Placenta The special effects of progesterone that are essential for the normal progression of pregnancy are as follows: • Progesterone causes decidual cells to develop in the uterine endometrium, and these cells play an important role in the

nutrition of the early embryo

• Progesterone decreases the contractility of the pregnant uterus, thus preventing uterine contractions from causing spontaneous abortion

• Progesterone contributes to the development of the conceptus even before implantation, because it increases the

secretions of the mother’s fallopian tubes and uterus to provide appropriate nutritive matter for the developing morula and blastocyst

• The progesterone secreted during pregnancy helps the estrogen prepare the mother’s breasts for lactation

Secretion of Estrogens by the Placenta • The syncytial trophoblast cells of the placenta secrete both estrogens and progesterone Function of Estrogen in Pregnancy • Causes enlargement of the mother’s uterus

• Causes enlargement of the mother’s breasts and growth of the breast ductal structure

• Causes enlargement of the mother’s female external genitalia. The estrogens also relax the pelvic ligaments of the

mother, so that the sacroiliac joints become relatively limber and the symphysis pubis becomes elastic during parturition

• Affect fetal development during pregnancy by affecting the rate of cell reproduction in the early embryo

Other Placental Hormones • Human placental fragments produce proopiomelanocortin (POMC)

• They also secrete GnRH and inhibin, and since GnRH stimulates and inhibin inhibits hCG secretion

• locally produced GnRH and inhibin may act in a paracrine fashion to regulate hCG secretion

• The trophoblast cells and amnion cells also secrete leptin, and moderate amounts of this satiety hormone enter the

maternal circulation

• The placenta also secretes prolactin in a number of forms. Finally, the placenta secretes the α subunits of hCG, and the plasma concentration of free α subunits rises throughout pregnancy

• These α subunits acquire a carbohydrate composition that makes them unable to combine with β subunits, and their prominence suggests that they have a function of their own

REFERENCES https://www.academia.edu/36004776/PRINCIPLES_OF_ANATOMY_AND_PHYSIOLOGY_Tortora_14th_Ed https://medicostimes.com/ganongs-review-physiology-pdf/ https://www.researchgate.net/publication/330486384_Guyton_and_Hall_Textbook_of_Medical_Physiology_-_12th-Ed