KELENJAR ENDOKRIN - HORMON TUBUH

The Endocrine System

I.    An Overview of the Endocrine System

1. A hormone is a chemical messenger secreted by one endocrine gland or cell into the bloodstream and targeted toward cells in another organ.  It’s another regulatory mechanism in addition to nervous system.
2. A hormone binds to specific receptors on plasma membranes or cytoplasm of its target cells.  Only those cells bear the specific receptors are affected.   Receptors of some hormones are found only in a limited number of cells while receptors of other could be distributed more extensively. 
3. The endocrine system is a collective term for all such hormone-secreting glands as well as the hormone-secreting cells located elsewhere.

II.  General Concepts about Hormones and Their Actions

      Hormones fall into three major chemical classes: steroids, biogenic amines, and peptide hormones.

Steroid hormones

1. Include estrogens, progesterone, androgens, glucocorticoids, and aldosterone
2. Derived from cholesterol
3. Plasma membrane permeable
4. Receptors located in the nucleus of their target cells
5. Activate DNA transcription

Biogenic amines

1. Include epinephrine, norepinephrine, dopamine, serotonin, melatonin, and thyroid hormones
2. Synthesized from amino acids
3. Plasma membrane impermeable (except thyroid hormones)
4. Receptor located in plasma membrane (except thyroid hormones)
5. Effects mediated through second-messenger systems

Peptide hormones

1. Include all the hormones that are not listed above.
2. Derived from proteins
3. Plasma membrane impermeable
4. Receptor located in plasma membrane

Hormone Receptors

1. Hormones stimulate only those cells that have specific receptors for them.
2. To each of many hormones, there can be several types and subtypes of receptors.
3. Different subtypes of the receptors mediate different effects.
4. The effects of each hormone on individual target cells are determined by the specific subtype of receptors that the cells carry.
5. Hormones bind to their receptors located either in the plasma membrane or in the nucleus, and activate the receptors. The activated receptors then turn on or off certain metabolic pathways. The intermediate processes between receptor activation and the final biological effects of the hormone, are called signal transduction.

Up- and Down-Regulation of Receptors

Up-regulation: 

• Long-term exposure to low levels of the hormone causes an increase in the number of receptors and sensitivity to the hormone.

Down-regulation (desensitization):

• Long-term exposure to high levels of the hormone causes a decrease in the number of receptors and sensitivity to the hormone.

Hormone Deactivation

• Hormones must be deactivated after carrying out their tasks. The liver performs this function in less than two minutes after they have gone into circulation in the bloodstream.

Individual Endocrine Glands

The Pituitary Gland /Hypofise

Anterior Lobe Hormones

The anterior pituitary synthesizes and secretes six hormones.

1. Follicle-stimulating hormone (FSH) stimulates follicle and egg development in the ovaries, and sperm production in the testes.
2. Luteinizing hormone (LH) stimulates female ovulation and growth of the corpus luteum. In males, LH stimulates the testes to secrete testosterone.
3. Thyroid-stimulating hormone (TSH) or thyrotropin stimulates the thyroid to produce and secrete its hormones.
4. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to secrete its hormones, as well as fat catabolism in adipose tissue and insulin secretion by the pancreas.
5. Prolactin (PRL) acts on the female mammary gland to promote milk synthesis. In males, it indirectly enhances the secretion of testosterone.
6. Growth hormone (GH), or somatotropin, stimulates cellular growth, mitosis, and differentiation, promoting overall tissue and organ growth.

FSH, LH, TSH, and ACTH are called tropic hormones because their targets also are endocrine glands. Secretion of the tropic hormones are subject to the feedback control of their target hormones in the blood.

Posterior Lobe Hormones

The posterior lobe stores and secretes two hormones manufactured in the hypothalamus.

1. Antidiuretic hormone (ADH) or vasopressin  : Two receptors, V1 and V2, mediates totally different effects. V1 receptors in vascular smooth muscles mediate vasoconstriction while V2 receptors in renal collecting ducts mediate an increase in water reabsorption.
2. Oxytocin (OT) stimulates uterine labor contractions and milk release by mammary glands.

Hypothalamus

The hypothalamus secretes both releasing hormones and inhibiting hormones to regulate the release of anterior pituitary hormones.

• They include thyrotropin-releasing hormone (TRH), corticotropin-releasing hormone (CRH), gonadotropin-releasing hormone (GnRH), prolactin-releasing factor (PRF), prolacting-inhibiting factor (PIF), growth hormone-releasing hormone (GHRH), and growth hormone-inhibiting hormone (GHIH).

Secretion of hypothalamic hormones is controlled normally via negative feedback inhibition by their target pituitary hormones.

The Pineal Gland

1. The pineal gland (epiphysis cerebri) is attached to the roof of the third ventricle. Its size regresses with age.
2. The pineal gland produces serotonin during the day, and melatonin at night. There may be a relationship between pineal gland and sexual maturation, mood, including depression and sleep disorders, and aging.

The Thymus

1.       The thymus, located in the mediastinum, undergoes involution after puberty.

2.       The thymus secretes thymopoietin and thymosins that regulate the development and activation of T-lymphocytes.

The Thyroid

1.       The thyroid, located below the larynx, is the largest endocrine gland, and receives the highest rate of blood flow per gram of tissue.

2.       Thyroid follicular cells secrete thryoxine (T4) and triiodothyronine (T3). Target cells convert T4 to T3. The effects of T3 on target cells increase the oxygen consumption and heat production in the cell, accounting for the calorigenic effect. This effect is stimulated by cold weather and pregnancy.

3.       C cells (parafollicular cells) secrete calcitonin, a hormone that lowers blood calcium levels by promoting calcium deposition in bone.

The Parathyroids

1.       The parathyroids (usually four) are located on the posterior of the thyroid.

2.       The parathyroids secrete parathyroid hormone (PTH) in response to hypocalcemia. PTH raises blood levels of calcium by promoting its absorption through the intestines, by inhibiting urinary excretion of calcium, and by increasing osteoclast activity. PTH reduces blood levels of phosphate.

The Adrenals

The Adrenal Medulla

·         The medulla secretes catecholamines (epinephrine and norepinephrine) in response to sympathetic stimulation. Their effects mimic those of the sympathetic nervous system but last longer because they are secreted into the bloodstream.

The Adrenal Cortex

1.         The adrenal cortex consists of three layers of modified epithelial cells: the outer zona glomerulosa, a middle zona fasciculata, and an inner zona reticularis.

2.         The cortex secretes more than 25 steroid hormones (corticosteroids) that fall into three categories: sex steroids (androgens, such as DHEA), mineralocorticoids (aldosterone), and glucocorticoids (cortisol).

3.         Glucocorticoid secretion, as stimulated by ACTH, stimulates fat and protein catabolism, and increases energy supplies in the bloodstream. These hormones are secreted in response to stress.

The Pancreas

1.         Insulin, from beta cells, is secreted in response to rising blood levels of glucose and other nutrients. Insulin promotes the synthesis of glycogen, fat, and protein, thus lowering blood glucose levels.

2.         Glucagon, from alpha cells, is secreted in response to low blood levels of glucose. It stimulates fat catabolism, glycogenolysis, and gluconeogenesis.

3.         Somatostatin (from delta cells) is chemically identical to GHIH, and is secreted by the pancreas as a paracrine secretion. It diffuses to alpha and beta cells and modulates their secretion of glucagon and insulin. 

The Gonads

1.       The gonads are both exocrine and endocrine.

2.       Ovarian follicles produce estradiol prior to menstruation, and progesterone afterward. The follicle secretes inhibin to prevent further release of FSH.

3.       Interstitial cells between seminiferous tubules secrete testosterone, which stimulates development of the male reproductive system and masculine physique. Sustentacular cells of the testes secrete inhibin, which also slows FSH release.

Endocrine Cells in Other Organs

1. The heart secretes atrial natriuretic factor (ANF) when overstretched by high blood pressure. This increases urine output to help lower blood pressure.
2. The kidneys produce calcitrol that affects the handling of calcium by the kidneys, small intestines, and bones, and erythropoietin that stimulates bone marrow to produce more red blood cells.
3. The liver secretes somatomedians, some of the body's erythropoietin, and a prohormone, angiotensinogen.
4. The stomach and small intestine produce enteric hormones that coordinate the activities of the digestive system.
5. The placenta secretes hormones during pregnancy that regulate the pregnancy and prime the mammary glands.