THE ENDOCRINE SYSTEM

GROUP 1By:Janine Kristel M. de Leon

• Endocrine glands• Ductless• Secrete hormones into the blood

• Hormones are regulatory molecules secreted into the body by endocrine glands

• Hormones affect the metabolism of their target organs and, by this means, help to regulate total body metabolism, growth and reproduction

Endocrine Glands and Hormones

Endocrine Gland

Major Hormones

Primary Target Organs

Primary Effects

Adipose tissue Leptin Hypothalamus Suppresses Appetite

Adrenal cortex Glucocorticoids

Aldosterone

Liver and Muscle

Kidneys

Glucocorticoids Influence glucose metabolism; aldosterone promotes Na+ retention, K+

excretion

Adrenal Medulla Epinephrine Heart, Brochioles, and blood vessels

Causes adrenergic stimulation

Heart Atrial natriuretic hormone

Kidneys Promotes excretion of Na+ in the urine

Hypothalamus Releasing and inhibiting hormones

Anterior pituitary

Regulates secretion of anterior pituitary hormones

Small intestine Secretin and cholecystokinin

Stomach, liver and pancreas

Inhibits gastric motility and stimulates bile and pancreatic juice secretion

Islets of Langerhans (pancreas)

Insulin

Glucagon

Many organs

Liver and adipose tissue

Insulin promotes cellular uptake of glucose and formation of glycogen and fat; glucagon stimulates hydrolysis of glycogen and fat

Kidneys Erythropoietin Bone marrow Stimulates red blood cell production

Liver Somatomedins Cartilage Stimulates cell division and growth

Ovaries Estradiol-17β and progesterone

Female reproductive tract and mammary gland

Maintains structure of reproductive tract and promotes secondary sex characteristics

Parathyroid glands

Parathyroid hormones

Bone, small intestine and kidneys

Increases Ca2+ concentration in blood

Pineal gland Melatonin Hypothalamus and anterior pituitary

Affects secretion of gonadotrophic hormones

Pituitary, anterior

Trophic hormones

Endocrine glands and other organs

Stimulates growth and development of target organs; stimulates secretion of other hormones

Pituitary, posterior

Antidiuretic hormone

Oxytocin

Kidneys and blood vesselsUterus and mammary glands

Antidiuretic hormone promotes water retention and vasoconstriction; oxytocin stimulates contraction of uterus and mammary secretory units

Skin 1,25-Dihydroxyvitamin D3

Small intestine Stimulates absorption of Ca2+

Stomach Gastrin Stomach Stimulates acid secretion

Testes Testosterone Prostate, seminal vesicles and other organs

Stimulates secondary sexual development

Thymus Thymopoietin Lymph nodes Stimulates WBC production

Throid Gland Throxine (T4) and triiodothyronine (T3); calcitonin

Most organs Throxine and triiodothyronine promotes growth and development and stimulates basal rate of cell respiration (basal metabolic rate or BMR); calcitonin may participate in the regulation in the blood Ca2+

levels

COMPARISON OF NERVOUS SYSTEM AND ENDOCRINE SYSTEM

CHARACTERISTIC

NERVOUS SYSTEM

ENDOCRINE SYSTEM

Mediator molecules

Neurotransmitter released locally in response to nerve response

Hormones delivered to tissues throughout the body by the blood

Site of mediator action

Close to site of release, at a synapse; binds to receptor in postsynaptic membrane

Far from site of release (usually) binds to receptors on or in target cells

Types of target cells

Muscle (smooth, cardiac and skeletal) cells, gland cells, other neurons

Cells throughout the body

Time to onset of action

Typically within millisecond (thousandths of a second)

Seconds to hours or days

Duration of action Generally briefer (milliseconds)

Generally longer (seconds to days)

HORMONE ACTIVITY

Hormones like Neurotransmitter, influence their target cells by chemically binding to specific protein receptors. Only the target cells for a given hormone have receptors that bind and recognize that hormone.

Receptors, like other cellular proteins, are constantly being

synthesized and broken down. Generally, a target cell has 2000

to 100,000 receptors for a particular hormone.

The Role of Hormone Receptors

If a hormone is present in excess, the number of target-cell receptors may

decrease—an effect called down-regulation. Down-regulation makes a target cell less sensitive to a hormone.

In contrast, when a hormone is deficient, the number of receptors may increase. This phenomenon, known as upregulation, makes a target cell more sensitive to a hormone.

CIRCULATING AND LOCAL HORMONES

Circulating hormones—they pass from the secretory cells that make them into interstitial fluid and then into the blood.

Local hormones, act locally on neighboring cells or on the same

cell that secreted them without first entering the bloodstream. • Paracrines (para- beside or near) - Local hormones that

act on neighboring cells• Autocrines - act on the same cell that secreted them (auto-

self ).

One example of a local hormone is interleukin 2 (IL-2), which is released by helper T cells (a type of white blood cell) during immune responses. IL-2 helps activate other nearby immune cells, a paracrine effect. But it also acts as an autocrine by stimulating the same cell that released it to proliferate.

Circulating and Local Hormones

• Local hormones usually are inactivated quickly; circulating hormones may linger in the blood and exert their effects for a few minutes or occasionally for a few hours. In time, circulating hormones are inactivated by the liver and excreted by the kidneys.

CHEMICAL CLASSIFICATION OF HORMONES

1. Amines• Hormones derived from amino acids tyrosine

and tryptophan• Include the hormones secreted by the adrenal

medulla, thyroid and pineal glands

2. Polypeptides and proteins• Polypeptide hormones contain less than 100

amino acids (ex. Antidiuretic hormone)• Protein hormones are polypeptides with more

than 100 amino acids (ex. Growth hormones

3. Glycoproteins• Consist of a long polypeptide (more

than 100 amino acids) bound to one or more carbohydrate groups (ex. FSH and LH)

4. Steroids• Lipids derived from cholesterol (ex.

Testosterone, estradiol, progesterone and cortisol)

Hormone Structure Gland Primary Effects

Antidiuretic hormone

8 amino acids Posterior pituitary

Water retention and vasoconstriction

Oxytocin 8 amino acids Posterior pituitary

Uterine and mammary contraction

Insulin 21 and 30 amino acids (double chain)

Beta cells in islets of Langerhans

Cellular glucose uptake, lipogenesis and glycogenesis

Glucagon 29 amino acids Alpha cells in islets of Langerhans

Hydrolysis of stored glycogen and fat

ACTH 39 amino acids Anterior pituitary

Stimulation of adrenal cortex

Parathyroid hormone

84 amino acids Parathyroid Increase in blood Ca2+

concentration

FSH, LH, TSH Glycoproteins Anterior pituitary

Stimulation of growth, development and secretory activity of target glands

Hormone molecules can be divided into those that are polar, and therefore water soluble, and those that are nonpolar, and thus insoluble in water. (in terms of their actions in target cells)• Lipophilic hormones – nonpolar hormones

soluble in lipids. They can gain entry into their target cells. These include the steroid hormones and thyroid hormones.

Steroid hormones are secreted only by 2 endocrine glands:• Adrenal Cortex – secrete corticosteroids (cortisol

and aldosterone) and small amounts of sex steroid• Gonads – secrete sex steroids

The major thyroid hormones are composed of two derivatives of the amino acid tyrosine bonded together.

• Tetraiodothyronine (T4) or throxine – contains 4 iodine atoms

• Triiodothyronine (T3) – contains 3 iodine atoms

The pineal gland secretes melatonin, hormone derived from the amino acid tryptophan

The adrenal medulla secretes the catecholamines epinephrine and norepinephrine which are derived from the amino acid tyrosine.

Endocrine Gland

Prehormone Active Products

Comments

Skin Vitamin D3 1,25-Dihydroxyvitamin D3

Hydroxylation reactions occur in the liver and kidneys

Testes Testosterone Dihydrotestosterone (DHT)

Estradiol-17β (E2)

DHT and other 5α-reduced androgens are formed in most androgen-dependent tissue

E2 is formed in the brain from testosterone, where it si beleived to affect both endocrine function and behavior; small amounts of E2 are also produced in the testes

Thyroid Thyroxine (T4) Triiodothronine (T3)

Conversion of T4 to T3 occurs in almost all tissues

Prohormones and Prehormones

Hormone molecules that affect the metabolism of target cells are often derived from less active “parent” or precursor, molecules.

Insulin for example is derived from proinsulin within the beta cells of islets of Langerhans of the pancreas.

In some cases, the prohormone itself is derived from an even larger precursor molecule; in the case of insulin, this molecule is calle preproinsulin.

Prehormone is used to indicate such precursors of prohormone.

The term prehormone designate those molecules secreted by endocrine glands that are inactive until changed by their target cells.

Common Aspects of Neural and Endocrine Regulation

Regardless of whether a particular chemical is acting as a neurotransmitter or as a hormone, in order for it to function in physiologic condition:(1) target cells must have specific receptor proteins that combine with the regulatory molecule;(2) the regulation of the regulatory molecule with the receptor proteins must cause a specific sequence of changes in the target cells; and(3) there must be a mechanism to quickly turn off the action of the regulator. This mechanism which involves rapid removal and/or chemical inactivation of the regulatory molecules, is essential because without an “off-switch” physiological control would be impossible.

Hormone Interaction

When two or more hormones work together to produce a particular result, their effects are said to be synergistic. These effects may be additive or complementary.

• Additive – action of the epinephrine and norepinephrine on the heart

• Complementary – action of FSH and testosterone

Synergistic and Permissive Effects

A hormone is said to have a permissive effect on the action of a second hormone when it enhances the responsiveness of a target organ to the second hormone or when it increases the activity of the second hormone-Estrogen has a permissive effect on the responsiveness of the uterus to progesterone-Glucocorticoids exert permissive effects on the actions of catecholamins-Parathyroid hormones has a permissive effect on the actions of Vitamin D3

The action of one hormone antagonize the effects of another.-Lactation during pregnancy (estrogen and prolactin)-Antagonism in the action of insulin and glucagon on adipose tissue

Antagonistic Effect

Effects of Hormone Concentrations on Tissue Response

The half-life of a hormone – the time required for the plasma concentration of a given amount of the hormone to be reduced to half its reference level – ranges from minutes to hours for most hormones (thyroid hormone however is for several days)

Normal tissue responses are produced only when the hormones are present within their normal, or physiological, range of concentrations.

When some hormones are taken in abnormally high, or pharmacological, concentrations, their effects may be different from those produced by lower, more physiologic, concentrations.

Variations in hormone concentration within the normal, physiological range can affect the responsiveness of target cells. This is due in part to the effects of the polypeptide and glycoprotein hormones on the number of their receptor proteins in target cells. More receptors may be formed in the target cells in response to particular hormones.

Sometimes also called upregulationExample: GnRH

Priming Effects

Subsequent exposure to the same concentration of the same hormone produces less of a target tissue response. This desensitization may be due to the fact that high concentrations of these hormones cause a decrease in the number of receptor proteins in their target cells – a phenomenon called DOWN REGULATION. (ex. Adipose cells and testicular cells)

Desensitization and Downregulation

In order to prevent desensitization from occuring under normal conditions, many polypeptide and glycoprotein hormones are secreted in spurts rather than continuously. This pulsatile secretion is an important aspect in the hormonal control in the reproductive system.

Pulsatile secretion of GnRH and LH is needed to prevent desensitization (in gonadal function).

THE END BOW.