Endocrine Glands & Secretion PDF

TYPE OF ENDOCRINE GLAND AND THEIR SECRETION Dr. Wassan.M. Husain 2nd stage Medical college  The pituitary gland and hypothalamus The hypothalamus is a structure of the diencephalon of the brain located anterior and inferior to th...

TYPE OF ENDOCRINE GLAND AND THEIR SECRETION Dr. Wassan.M. Husain 2nd stage Medical college  The pituitary gland and hypothalamus The hypothalamus is a structure of the diencephalon of the brain located anterior and inferior to the thalamus. It has both neural and endocrine functions, producing and secreting many hormones. In addition, the hypothalamus is anatomically and functionally related to the pituitary gland (small, bean-shaped gland situated at the base of brain).  Hypothalamic releasing and inhibitory hormones that control secretion of the anterior pituitary gland: 1.Thyrotropin-Releasing Hormone (TRH): Stimulates secretion of TSH from anterior pituitary gland. 2. Gonadotropin-Releasing Hormone (GnRH): Stimulates secretion of FSH and LH by anterior pituitary gland. 3. Corticotropin-Releasing Hormone (CRH): Stimulates secretion of ACTH by anterior pituitary gland. 4. Growth Hormone–Releasing Hormone(GHRH): Stimulates secretion of growth hormone by anterior pituitary gland (somatotropin). 5. Growth Hormone Inhibitory Hormone (GHIH): Inhibits secretion of growth hormone by anterior pituitary gland (somatostatin). 6. Prolactin-Releasing Hormone (PRH): Stimulate secretion of prolactin by adenohypophysis. 7. Prolactin-Inhibiting Hormone (PIH): Inhibits secretion of prolactin by adenohypophysis  The pituitary gland The pituitary gland , also called the hypophysis, is a small gland— about 1 centimeter in diameter and 0.5 to 1 gram in weight—that lies in the sella turcica. Physiologically, the pituitary gland is divisible into two distinct portions: 1. The anterior pituitary, also known as the adenohypophysis, 2. The posterior pituitary, also known as the neurohypophysis. 3. Between these portions is a small, relatively avascular zone called the pars intermedia, which is much less developed in the human being but is larger and much more functional in some animals.  Six major peptide hormones are secreted by the anterior pituitary, and two important peptide hormones are secreted by the posterior pituitary. The hormones of the anterior pituitary play major roles in the control of metabolic functions throughout the body,  Pituitary Hormones Growth hormone (GH): promotes growth of the entire body by affecting protein formation, cell multiplication, and cell differentiation. Adrenocorticotropin (corticotropin)(ACTH) controls the secretion of some of the adrenocortical hormones, which affect metabolism of glucose, proteins, and fats. Thyroid-stimulating hormone (thyrotropin)(TSH) controls the secretion rate of thyroxine and triiodothyronine by the thyroid gland, and these hormones control the rates of most intracellular chemical reactions in the body. Prolactin promotes mammary gland development and milk production. Two separate gonadotropic hormones (GnRH), follicle stimulating hormone FSH and luteinizing hormone LH, control growth of the ovaries and testes, as well as their hormonal and reproductive activities. The two hormones secreted by the posterior pituitary play other roles: Antidiuretic hormone (ADH) (also called vasopressin) controls the rate of water excretion into the urine, thus helping to control the concentration of water in the body fluids. Oxytocin (OT) helps express milk from the glands of the breast to the nipples during suckling and helps in the delivery of the baby at the end of gestation.  Anterior pituitary The anterior pituitary is a highly vascular gland with extensive capillary sinuses among the glandular cells. Almost all the blood that enters these sinuses passes first through another capillary bed in the lower hypothalamus. The blood then flows through small hypothalamic-hypophysial portal blood vessels into the anterior pituitary sinuses. Small arteries penetrate into the median eminence and then additional small vessels return to its surface, coalescing to form the hypothalamic-hypophysial portal blood vessels. These vessels pass downward along the pituitary stalk to supply blood to the anterior pituitary sinuses.  Posterior pituitary The posterior pituitary is actually an extension of the neurons of the paraventricular and supraoptic nuclei of the hypothalamus. The cell bodies of these regions rest in the hypothalamus, but their axons descend as the hypothalamic–hypophyseal tract within the infundibulum, and end in axon terminals that comprise the posterior pituitary The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones produced by the hypothalamus. Paraventricular nuclei produce the hormone oxytocin, Supraoptic nuclei produce ADH. These hormones travel along the axons into storage sites in the axon terminals of the posterior pituitary. In response to signals from the same hypothalamic neurons, the hormones are released from the axon terminals into the bloodstream.  Neurohypophysis hormones Oxytocin: Antidiuretic Hormone (ADH)  Polypeptide hormone.  Polypeptide hormone.  Oxytocin is formed primarily in the paraventricular  ADH is formed primarily in the supraoptic nuclei nuclei  Blood osmolarity is constantly monitored by osmoreceptors—specialized cells  stimulates uterine contractions and dilation of the cervix. within the hypothalamus that are particularly sensitive to the concentration of  Throughout most of pregnancy, oxytocin hormone sodium ions and other solutes. receptors are not expressed at high levels in the uterus. Toward the end of pregnancy, the synthesis of oxytocin In the presence of ADH, the permeability of the collecting receptors in the uterus increases, and the smooth muscle ducts and tubules to water increases greatly and allows most of the water to be cells of the uterus become more sensitive to its effects. reabsorbed as the tubular fluid passes through these ducts, thereby conserving  Oxytocin is continually released throughout childbirth water in the body and producing very concentrated urine. through a positive feedback mechanism.  Oxytocin is necessary for the milk ejection reflex Without ADH, the luminal membranes of collecting ducts are almost impermeable to water. However, immediately inside the cell membrane (commonly referred to as “let-down”) in breast feeding are a large number of special vesicles that have highly water-permeable pores women. called aquaporins.  As the newborn begins suckling, sensory receptors in the  When ADH acts on the cell, it first combines with membrane receptors that nipples transmit signals to the hypothalamus. In response, activate adenylyl cyclase and cause the formation of cAMP inside the oxytocin is secreted and released into the bloodstream. tubular cell cytoplasm.  Cells in the mother’s milk ducts contract, ejecting milk  Phosphorylation of elements in the special vesicles, which then causes the into the infant’s mouth. Secondly, in both males and vesicles to insert into the apical cell membranes, thus providing many areas of females, oxytocin is thought to contribute to parent– high water permeability. newborn bonding, known as attachment.  This process temporarily provides many new pores that allow free diffusion of  Oxytocin is also thought to be involved in feelings of love water from the tubular fluid through the tubular epithelial cells and into the and closeness. renal interstitial fluid.  Regulation of antidiuretic hormone production 1. Increased Extracellular Fluid Osmolarity Stimulates ADH Secretion: concentrated body fluids stimulate the supraoptic nuclei, whereas dilute body fluids inhibit them. A feedback control system is available to control the total osmotic pressure of the body fluids. 2. Low Blood Volume and Low Blood Pressure Stimulate ADH Secretion—Vasoconstrictor Effects of ADH: Higher concentrations of ADH have a potent effect of constricting the arterioles throughout the body and therefore increasing the arterial pressure. For this reason,ADH has another name, vasopressin. 3. Decreased blood volume causing intense ADH secretion through the following: The atria have stretch receptors that are excited by overfilling. When excited, they send signals to the brain to inhibit ADH secretion. Conversely, when the receptors are unexcited as a result of under filling, the opposite occurs, with greatly increased ADH secretion. Decreased stretch of the baroreceptors of the carotid, aortic, and pulmonary regions also stimulates ADH secretion.  Anterior pituitar y hor mones All the major anterior pituitary hormones, except for growth hormone, exert their principal effects mainly by stimulating target glands, including thyroid gland, adrenal cortex, ovaries, testicles, and mammary glands. The functions of each of these pituitary hormones are so intimately concerned with the functions of the respective target glands that, except for growth hormone. Growth hormone, however, does not function through a specific target gland but exerts its effects directly on all or almost all tissues of the body.  Physiological functions of growth hormone Growth hormone (Somatotropic hormone or Somatotropin) 1- Growth hormone promotes growth of many body tissues: It causes growth of almost all tissues of the body that are capable of growing. It promotes increased sizes of the cells and increased mitosis, with development of greater numbers of cells and specific differentiation of certain types of cells such as bone growth cells and early muscle cells. 2- Growth hormone has several metabolic effects: (1) increased rate of protein synthesis in most cells of the body. (2) increased mobilization of fatty acids from adipose tissue, increased free fatty acids in the blood, and increased use of fatty acids for energy. (3) decreased rate of glucose utilization throughout the body. Thus, in effect, growth hormone enhances body protein, decreases fat stores, and conserves carbohydrates. 3- Growth Hormone Promotes Protein Deposition in Tissues by: A. Enhancement of amino acid transport through the cell membranes to the interior of the cells. B. Enhancement of RNA translation to cause protein synthesis by the ribosomes C. Increased nuclear transcription of DNA to form RNA. D. Decreased catabolism of protein and amino acids. Growth hormone enhances almost all facets of amino acid uptake and protein synthesis by cells, while at the same time reducing the breakdown of proteins. 4- Growth Hormone Enhances Fat Utilization for Energy: Growth hormone causing release of fatty acids from adipose tissue and, therefore, increasing the concentration of fatty acids in body fluids. Also, in tissues throughout the body, growth hormone enhances conversion of fatty acids to acetyl coenzyme A (acetyl -CoA) and its subsequent utilization for energy. Therefore, under the influence of growth hormone, fat is used for energy in preference to use of carbohydrates and proteins.  5- “Ketogenic” Effect of Excessive Growth Hormone. Under the influence of excessive amounts of growth hormone, fat mobilization from adipose tissue sometimes becomes so great that large quantities of acetoacetic acid are formed by the liver and released into the body fluids, thus causing ketosis. This excessive mobilization of fat from the adipose tissue also frequently causes a fatty liver. 6- Growth Hormone Decreases Carbohydrate Utilization Growth hormone causes multiple effects that influence carbohydrate metabolism, including: (1) Decreased glucose uptake in tissues such as skeletal muscle and fat. (2) Increased glucose production by the liver. (3) Increased insulin secretion. Each of these changes results from growth hormone induced “insulin resistance,” which attenuates insulin’s actions to stimulate uptake and utilization of glucose in skeletal muscle and adipose tissue and to inhibit gluconeogenesis by the liver This leads to increased blood glucose concentration and a compensatory increase in insulin secretion. For these reasons, growth hormone’s effects are called diabetogenic, and excess secretion of growth hormone can produce metabolic disturbances similar to those found in patients with type II diabetes, who are also resistant to the metabolic effects of insulin.  7- Growth hormone stimulates cartilage and bone growth Growth hormone stimulates increase growth of the skeletal frame. This results from multiple effects of growth hormone on bone, including: (1) Increased deposition of protein by the chondrocytic and osteogenic cells that cause bone growth. (2) Increased rate of reproduction of these cells. (3) A specific effect of converting chondrocytes into osteogenic cells, thus causing deposition of new bone.  There are two principal mechanisms of bone growth. First, in response to growth hormone stimulation, the long bones grow in length at the epiphyseal cartilages, where the epiphyses at the ends of the bone are separated from the shaft. This growth first causes deposition of new cartilage, followed by its conversion into new bone, thus elongating the shaft and pushing the epiphyses farther and farther apart. At the same time, the epiphyseal cartilage is progressively used up, so by late adolescence, no additional epiphyseal cartilage remains to provide for further long bone growth. At this time, bony fusion occurs between the shaft and the epiphysis at each end, so no further lengthening of the long bone can occur. Second, osteoblasts in the bone periosteum and in some bone cavities deposit new bone on the surfaces of older bone. Simultaneously, osteoclasts in the bone remove old bone. When the rate of deposition is greater than that of resorption, the thickness of the bone increases. Growth hormone strongly stimulates osteoblasts. Therefore, the bones can continue to become thicker throughout life under the influence of growth hormone; this is especially true for the membranous bones. For instance, the jaw bones can be stimulated to grow even after adolescence, causing forward protrusion of the chin and lower teeth. Likewise, the bones of the skull can grow in thickness and give rise to bony protrusions over the eyes.  8- Growth hormone exerts much of its effect through intermediate substances called somatomedins: Growth hormone causes the liver (and, to a much less extent, other tissues) to form several small proteins called somatomedins that have the potent effect of increasing all aspects of bone growth. Many of the somatomedin effects on growth are similar to the effects of insulin on growth. Therefore, the somatomedins are also called insulin-like growth factors (IGFs).  Regulation of growth hormone secretion The major long term controller of growth hormone secretion is the long-term state of nutrition of the tissues, especially their level of protein nutrition. That is, nutritional deficiency or excess tissue need for cellular proteins for instance, after a severe bout of exercise when the muscles’ nutritional status has been taxed in some way increases the rate of growth hormone secretion. Growth hormone, in turn, promotes synthesis of new proteins while at the same time conserving the proteins already present in the cells  Abnormalities of growth hormone secretion Panhypopituitarism: Decreased secretion of all the anterior pituitary hormones. The decrease in secretion may be congenital or it may occur suddenly or slowly at any time during life, most often resulting from a pituitary tumor that destroys the pituitary gland. in the Adult, first occurring in adulthood frequently results from one of three common abnormalities. Two tumorous conditions, craniopharyngiomas or chromophobe tumors, may compress the pituitary gland until the functioning anterior pituitary cells are totally or almost totally destroyed. The third cause is thrombosis of the pituitary blood vessels. The general effects of adult Panhypopituitarism are: (1)Hypothyroidism (2) Depressed production of glucocorticoids by the adrenal glands (3)Suppressed secretion of the gonadotropic hormones so that sexual functions are lost. Dwarfism: Result from deficiency of anterior pituitary secretion during childhood. In general, all the physical parts of the body develop in appropriate proportion to one another, but the rate of development is greatly decreased. A child who has reached the age of 10 years may have the bodily development of a child aged 4 to 5 years, and the same person at age 20 years may have the bodily development of a child aged 7 to 10 years. A person with panhypopituitary dwarfism does not pass through puberty and never secretes sufficient quantities of gonadotropic hormones to develop adult sexual functions. In one third of such dwarfs, however, only growth hormone is deficient; these persons do mature sexually and occasionally reproduce. In one type of dwarfism, the rate of growth hormone secretion is normal or high, but there is a hereditary inability to form somatomedin C, which is a key step for the promotion of growth by growth hormone. Treatment with Human Growth Hormone Human growth hormone can now be synthesized by Escherichia coli bacteria as a result of successful application of recombinant DNA technology. Therefore, this hormone is now available in sufficient quantities for treatment purposes. Dwarfs who have pure growth hormone deficiency can be completely cured if treated early in life. Human growth hormone may also prove to be beneficial Acromegaly: Acromegaly is a rare but serious medical condition that happens when you have too much growth hormone in your body. Acromegaly affects your body’s bones and tissues and causes them to grow in irregular ways. Causes tumor in your pituitary gland called a pituitary adenoma (acidophilic tumor) occurs after adolescence that is, after the epiphyses of the long bones have fused with the shafts the person cannot grow taller, but the bones can become thicker and the soft tissues can continue to grow. Adults with acromegaly may experience the following symptoms: Enlarged hands or feet. Changes in your face shape, including a more prominent jaw and/or forehead. Headaches. Increase in size of your lips, nose and/or tongue. Joint pain. Excessive sweating or oily skin. Vision changes. Deepening of your voice. Increase in the number of skin tags. Numbness in your hands. Sleep apnea. Carpal tunnel syndrome or spinal cord issues.  Role of decreased growth hormone secretion in causing changes associated with aging In people who have lost the ability to secrete growth hormone, some features of the aging process accelerate. For instance, a 50-year-old person who has been without growth hormone for many years may have the appearance of a person aged 65 years. The aged appearance seems to result mainly from decreased protein deposition in most tissues of the body and increased fat deposition in its place. The physical and physiological effects are increased wrinkling of the skin, diminished rates of function of some of the organs, and diminished muscle mass and strength Growth hormone therapy in older people have demonstrated three important beneficial effects: (1) Increased protein deposition in the body, especially in the muscles. (2) Decreased fat deposits. (3) A feeling of increased energy. Undesirable adverse effects of Growth hormone therapy including (1) Insulin resistance and diabetes (2) Edema (3) Carpal tunnel syndrome (4) Arthralgias (joint pain). Therefore, recombinant growth hormone therapy is generally not recommended for use in healthy elderly patients with normal endocrine function.