endocrinology

Hormones are organic substances, produced in small amounts by specific tissues (endocrine glands), secreted into the blood stream to control the metabolic and biological activities in the target cells.

Hormones may be regarded as the chemical messengers involved in the transmission of information from one tissue to another and cell to cell.

Endocrine hormones: produced by ductless endocrine glands mix to blood have target organs e.g. TH, LH, FSH

Autocrine hormones: act on the same cells where they are synthesized.

e.g. interleukin-2 Paracrine hormones:

act on the cells adjacent/close to the cells from where they are synthesized e.g. prostaglandins

Neurotransmitters: released by nerve cells and usually act on the

adjacent cells. e.g. Catecholamines

Hypothalamus Pituitary Thyroid Adrenal Gonads: Ovaries & Testes

1. BASED ON CHEMICAL NATURE:

Peptide hormones : Insulin Glucagon ADH

Oxytocin

Steroid hormones: Sex hormones Glucocorticoids Mineralocorticoids

Amino acid derivatives: Epinephrine Norepinephrine Thyroid hormones

Two divisions:

Anterior pituitary(adenohypophysis)

Posterior pituitary(neurohypophysis)

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Sits in hypophyseal fossa: depression in sella turcica of sphenoid bone

Pituitary secretes 9 hormonesThe Pituitary

1. TSH2. ACTH3. FSH4. LH5. GH6. PRL7. MSH

8. ADH (antidiuretic hormone), or vasopressin9. Oxytocin

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The first four are “tropic” hormones, they regulate the function of other hormones

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2. BASED ON MECHANISM OF ACTION

GROUP I: intracellular receptor (Lipophilic)▪ Hormones bind to intracellular receptors (HRE in DNA)

to form receptor hormone complexes to carry out the biochemical functions. They are derivatives of cholesterol, lipophilic in nature and possess long half lives.

GROUP II: surface receptor (Hydrophilic) These hormones bind to cell surface receptors and

stimulate the release of second messengers which in turn perform the biochemical function. They are hydrophilic in nature and possess short half lives.

Hormones bind with HRE in DNA Estrogens Progestins Androgens Glucocorticoids Mineralocorticoids Calcitriol Thyroid Hormones

(a) The second messenger is cAMP Adrenocorticotropic hormone (ACTH) Follicle stimulating hormone (FSH) Leuteinzing Hormone (LH) Thyroid stimulating hormone (TSH) Chorionic gonadotropins (hCG) β Endorphins & Enkephalins Anti diuretic hormone (ADH) Glucagon Parathyroid hormone (PTH) Calcitonin Epinephrine Nor-epinephrine

(b) The second messenger is cGMP Atrial natriuretic factor (ANF) Nitric oxide (NO)

(c) The second messenger is phosphotidyl inositol/calcium (or both)

Thyrotropin releasing hormone (TRH) Gonadotropin releasing hormone (GnRH) Gastrin Cholecystokinin (CCK)

(d) The second messenger is unknown/ unsettled (Kinase/Phosphatase cascade)

Insulin Growth hormone (GH) Prolactin (PRL) Oxytocin Somatomedins (Insulin like growth factor) (IGF-

I, IGF-II)

Group I Hormones Lipophilic in nature → cross plasma membrane by

diffusion Act through intracellular receptors located either in

cytosol or nucleus Duration of action is hours to days Hormone first binds with receptor → forms HR-

complex → binds with the specific region on the DNA called hormone responsive element (HRE) → causes increased expression of specific genes → transcription → translation → production of specific proteins → carries biochemical action of hormone

HORMONE RESPONSE ELEMENT (HRE):- Specific sequence present in DNA which binds HR

complex during lipophilic hormone action HRE hormone specific

RESULT:- Binding of HR complex to HRE → transcription →

mRNA → Translation (specific protein) → Biochemical response

STRUCTURE OF RECEPTOR MOLECULES Large, integral membrane proteins with

specificity & high affinity for hormones. Hormone & receptor binding is reversible. Initiate response without entering the cell. Rapid response/ shorter duration of action

(seconds to hours). Mediate effect through second messengers.

GROUP II HORMONES These hormones are considered as first

messenger Hormone binds to surface receptors Carry the action through mediatory molecules

(second messengers) Hormones → First messenger → Second

messenger → Activation of protein kinase → phosphorylation of enzyme → biochemical action.

Cyclic adenosine 3’,5’-monophosphate (cAMP) consists of adenine, ribose & a phosphate.

Adenylate cyclase converts ATP to cAMP cAMP is a second messenger for majority of

polypeptide hormones cAMP binds with protein kinase and activates it to

carry out the phosphorylation for the biochemical action of hormone

cAMP degrades to 5’ AMP by phosphodiasterase

Figure 18.3 G Proteins and Hormone Activity

Figure 18.8 Feedback control of Endocrine Secretion

Similarities between neural and endocrine pathways

(a) Humoral: in response to changing levels of ions or nutrients in the blood

(b) Neural: stimulation by nerves(c) Hormonal: stimulation received from

other hormones

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