Introduction to Endocrinology Prof. dr. Zoran Valić Department of Physiology University of Split School of Medicine

Coordination of Body Functions nervous system (neurotransmitters into the synaptic junctions – locally) endocrine system (hormones into blood) neuroendocrine (neurohormones into blood) paracrines (secreted into extracellular fluid and affect neighboring different target cells) autocrines (affect same cells) cytokines (peptides - 5,4 or 2 - interleukins, lymphokines, adipokines (leptin))

some endocrine hormones affect many different types of cells of the body (growth hormone, thyroxine) other hormones affect mainly specific target tissues (ACTH – adrenal cortex) regulation: metabolism, growth and development, water and electrolyte balance, reproduction, and behavior

Chemical Structure and Synthesis proteins and polypeptides steroids derivatives of the amino acid tyrosine (thyroid and the adrenal medullae) there are no known polysaccharides or nucleic acid hormones

1) Protein Hormones widespread, water soluble from 3 (TRH) to 200 amino acids (growth hormone and prolactin) more than100 amino acids – proteins synthesized on the rough end of the ER preprohormones – prohormones (ER) – hormones (Golgi apparatus & secretory vesicles) – exocytosis (Ca, cAMP)

2) Steroid Hormones usually synthesized from cholesterol are not stored lipid soluble most of cholesterol comes from plasma, but there is also de novo synthesis consist of three cyclohexyl rings and one cyclopentyl ring combined into a single structure

3) Amine Hormones derived from tyrosine most of the thyroid hormones combine with plasma proteins 4x more epinephrine than norepinephrine taken up into preformed vesicles and stored until secreted (exocytosis) in the plasma in free form or in conjugation with other substances

Hormone Secretion some hormones (epinephrine) are secreted within seconds after stimulation, and develop action within seconds to minutes thyroxine or growth hormone may require months for full effect concentrations of hormones are incredibly small (from 10-12 to 10-6g/ml) – rates of secretion are extremely small (μg-mg/day)

Control of Hormone Secretion in most instances – negative feedback mechanisms controlled variable is sometimes not the secretory rate of the hormone itself but the degree of activity of the target tissue

regulation of gene transcription and translation steps involved in the synthesis of hormones and steps involved in processing hormones or releasing stored hormones positive feedback – LH, oxytocin cyclical variations – seasonal changes, various stages of development and aging, the diurnal (daily) cycle, and sleep

Transport of Hormones in Blood water-soluble hormones (peptides and catecholamines) – dissolved in the plasma steroid and thyroid hormones – circulate in the blood mainly bound to plasma proteins (usually less than 10% free in solution), biologically inactive (serve as reservoirs, greatly slows their clearance from the plasma)

Clearance of Hormones from Blood concentration of a hormone in the blood: rate of hormone secretion into the blood rate of removal of the hormone from the blood (metabolic clearance rate – number of milliliters of plasma cleared of the hormone per minute) one measures : the rate of disappearance of hormone from the plasma & concentration

metabolic destruction by the tissues binding with the tissues excretion by the liver into the bile excretion by the kidneys into the urine liver damage  excessively high concentration of steroid hormones half-life of angiotensin II < 1 minute, thyroid hormones 1-6 days

Hormone Receptors and Their Activation binding to specific receptors at the target cell – first step of a hormone's action initiation of a cascade of reactions in the cell – amplification of the effect hormonal receptors are large proteins, each cell usually has some 2000 to 100,000 receptors receptor is highly specific for a single hormone

in or on the surface of the cell membrane (protein, and catecholamine hormones) in the cell cytoplasm (steroid hormones) in the cell nucleus (thyroid hormones) number of receptors usually does not remain constant (increase or decrease)

Intracellular Signaling formation a hormone-receptor complex – alteration of function of receptor : ion channel-linked receptors G protein-linked hormone receptors enzyme-linked hormone receptors intracellular hormone receptors and activation of genes

1) Ion Channel-Linked Receptors acetylcholine & norepinephrine change in the structure of the receptor opening or closing a channel for one or more ions (Na, K, Ca) few directly, most indirectly by coupling with G protein-linked or enzyme-linked receptors

2) G Protein-Linked Hormone Receptors heterotrimeric GTP-binding proteins > 1000 known G protein-coupled receptors all have 7 transmembrane segments that loop in and out of the cell membrane cytoplasmic tail is coupled to G protein (include three parts – α, β, and γ subunits) binding of hormone – conformational change in receptor – activation of G protein

G proteins binds GDP (α subunit) activated G proteins open or close cell membrane ion channels change the activity of an enzyme in the cytoplasm G proteins binds GDP (α subunit) displacement of GDP by GTP causes the α subunit to dissociate from the trimeric complex and to associate with other intracellular signaling proteins inhibitory (Gi) & stimulatory (Gs) proteins

3) Enzyme-Linked Hormone Receptors some receptors, when activated, function directly as enzymes or are closely associated with enzymes that they activate pass through the membrane only once hormone-binding site on the outside of the cell membrane

catalytic or enzyme-binding site on the inside leptin receptor member of a large family of cytokine receptors that do not themselves contain enzymatic activity but signal through associated enzymes one of the signaling pathways occurs through a tyrosine kinase of the janus kinase (JAK) family, JAK2

4) Intracellular Hormone Receptors and Activation of Genes steroid hormones, thyroid hormones, retinoid hormones, and vitamin D receptors in the cytoplasm or nucleus binding with a specific regulatory (promoter) sequence of the DNA – hormone response element transcription of specific genes and formation of mRNA

Second Messenger Mechanisms cAMP, cGMP calcium ions and associated calmodulin 4 binding sites for Ca, 3 or 4 changes its shape activation/inactivation of protein kinases phosphorylation of proteins (myosin light chain kinase – smooth muscle contraction) products of membrane phospholipid breakdown

could be Gi amplification of the effect – cascade of reactions

phosphatidylinositol biphosphate contraction, secretion

Steroid Hormones Increase Protein Synthesis these proteins then function as enzymes, transport proteins, or structural proteins aldosterone – for 45 minutes, protein which promote sodium reabsorption and potassium secretion full action is delayed for at least 45 minutes-up to several hours or even day

Thyroid Hormones Increase Gene Transcription in the Cell Nucleus thyroxine and triiodothyronine bind directly with receptor proteins in the nucleus (activated transcription factors located within the chromosomal complex) they control the function of the gene promoters

formation of many types of intracellular proteins thyroid hormones can continue to express their control functions for days or even weeks

Measurement of Hormone Concentrations in the Blood extremely minute quantities (pg/mL) radioimmunoassay production of antibody that is highly specific for the hormone to be measured is produced small quantity of this antibody (smaller than total possible) is mixed with: sample containing the hormone to be measured appropriate amount of purified standard hormone that has been tagged with a radioactive isotope error of 10 to 15 percent

Measurement of Hormone Concentrations in the Blood ELISA (enzyme-linked immunosorbent assay) it does not employ radioactive isotopes much of the assay can be automated using 96-well plates cost-effective and accurate method for assessing hormone levels