Pituitary Hormones I PDF

Pituitary hormones: I growth hormone Learning Outcomes By the end of the lecture, the students will be able to: 1. Recognize the role of growth hormone in growth control. 2. Recognize the metabolic functions of growth hormone. 3. Enumerate the factors that control growth hormone secretion 4. Un...

Pituitary hormones: I growth hormone Learning Outcomes By the end of the lecture, the students will be able to: 1. Recognize the role of growth hormone in growth control. 2. Recognize the metabolic functions of growth hormone. 3. Enumerate the factors that control growth hormone secretion 4. Understand the control of growth hormone secretion. Describe the abnormalities of growth hormone secretion References Guyton & Hall textbook of Medical Physiology. Elsevier, 14th Edition 2021. USMLE STEP I Lecture notes 2017 Physiology. KAPLAN Medical  The role of growth hormone in growth control. Growth hormone regulates growth and metabolism. It is secreted by somatotrophs of the anterior pituitary Also called somatotropic hormone or somatotropin Intrauterine Growth Growth hormone and insulin are responsible for intrauterine growth. Infants of diabetic mothers are born larger than infants of non diabetic mothers because they have increased insulin levels during intrauterine life. Postnatal Growth Growth hormone, insulin, and thyroid hormones are the main hormones responsible for postnatal growth..  Actions of Growth hormone Growth hormone exerts its effects both directly and indirectly. Direct effects occur when the hormone acts on its specific receptors Indirect effects are mediated by stimulating the secretion of another hormone secreted by the liver called insulin-like growth factor-1 (IGF-1). IGF-1 is a member of a group of hormones called somatomedins formed in the liver under the influence of growth hormone. It is a circulating growth factor similar in structure to proinsulin and has some insulin-like activity. It circulates in the blood tightly bound to a large protein, Protein binding increases the half-life Growth effects of growth hormone (indirect effect) IGF-1 stimulates the growth of all tissues capable of growing by stimulating: (a) proliferation (b) differentiation of cells. A) It stimulates bone growth by stimulating proliferation and differentiation of osteoblast and chondrocyte and increases their activity. B) It stimulates conversion of chondrocytes into osteogenic cells, causing deposition of new bone. B) It stimulates muscle growth, by promoting the differentiation and proliferation of the myoblasts into a muscle cell There are two principal mechanisms of bone growth: First, the long bones grow in length at the epiphyseal cartilages by deposition of new cartilage, followed by its conversion into new bone, thus elongating the shaft. By late adolescence, bony fusion occurs between the shaft and the epiphysis and no further long bone growth occurs. Second, osteoblasts on the bone surface and lining some bone cavities deposit new bone on the surfaces and cavities of older bone At the same time, osteoclasts remove old bone. When the rate of deposition is greater than rate of removal, the thickness of the bone increases. Growth hormone strongly stimulates osteoblasts. Therefore, the bones continue to become thicker throughout life especially the membranous bones. Lo2: The metabolic functions of growth hormone (direct action). 1. Lipid metabolism: Growth hormone promotes the use of lipids for energy production by promoting the breakdown of triglycerides (lipolysis) and oxidation of free fatty acids in cells. "Ketogenic" Effect of Excessive Growth Hormone: In case of excessive amounts of growth hormone, fat mobilization from adipose tissue becomes so great that large quantities of acetoacetic acid are formed by the liver and released into the body fluids, causing ketosis. 2. Protein metabolism: Growth hormone has the following effects: 1. Enhances the cellular uptake of amino acids and protein synthesis, and decreases protein oxidation ( the net effect is protein anabolism). 2. Enhancement of RNA Translation to cause Protein Synthesis by the Ribosomes. 3. Increased Nuclear Transcription of DNA to Form RNA. 3. Carbohydrate metabolism: a. Growth hormone has anti-insulin activity. It suppresses insulin’s action by decreasing the sensitivity of insulin receptors. b. It also increases gluconeogenesis in the liver. The net result is to increases blood glucose which leads to increased insulin secretion. Excessive growth hormone causes changes like diabetes type II Lo3: Factors that affect growth hormone secretion 1. Growth hormone is secreted during the period of growth. After adolescence, secretion decreases slowly with aging. 2. Secretion of GH requires the normal plasma levels of thyroid hormones. GH secretion is markedly reduced in hypothyroid individuals. 4. The following factors factors related to state of nutrition or stress affects growth hormone secretion. Stimuli that 1.Hypoglyce increase GH 2.Exercise mia secretion 3.Stress 4. Sleep 5.Fasting 6.Increase in circulating 7.Protein levels of meal 8.Glucagon amino acids like arginine 9.ghrelin 10.Androgens  1.Increased blood glucose 2.Increased blood free fatty acids Stimuli 3.Aging that 4.Obesity decreas 5.Growth hormone e GH inhibitory hormone (somatostatin) secretio 6.Exogenous growth n hormone 7.Somatomedins (insulin- like growth factors) Pattern of secretion of Growth hormone Lo4: Control of growth hormone secretion. Hypothalamic Control of Growth Hormone Secretion 1. Hypothalamic growth hormone-releasing hormone (GHRH) increases growth hormone secretion. 2. Ghrelin hormone is found in hypothalamus and stimulate GH secretion. Ghrelin is involved in the regulation of food intake. 3. Emotions, stress, and trauma can all affect hypothalamic control of growth hormone secretion. 4. Hypothalamic growth hormone inhibiting factor (somatostatin) decreases GH secretion. 5. IGF-I exerts a –ve feed back on GH secretion as it acts on hypothalamus increases the secretion of somatostatin. Lo1: DISORDERS OF GROWTH HORMONE : 1- Deficiency of GH: a) In adults: No syndrome, only a tendency to hypoglycemia. b) In children: (pituitary dwarfism) In case of panhypopituitarism, the dwarf does not pass through puberty and never develop adult sexual functions (a condition known as infantilism). In cases when only growth hormone is deficient; these persons can reach sexual maturation. 2- Increase the secretion of growth hormone (due to neoplasm) A) Gigantism: In this condition the growth hormone is increased before the closure of epiphysis. All body tissues grow rapidly, including the bones, height increases so that the person becomes a giant up to 8 feet tall. The giant ordinarily has hyperglycemia. B) Acromegaly: In this condition the growth hormone is increased after puperty. It is almost always due to macroadenoma (> 1 cm diameter) of the anterior pituitary There is enlargement of the hands and feet, and a protrusion of the lower jaw (prognathism). Overgrowth of frontal, and facial bones (acromegalic facies). About 25% of patients have abnormal glucose tolerance tests, and 4% develop diabetes