What are the three types of chemical messengers?

Chemical messengers

Chemical messengers

Describes the different types of chemical messengers in mammalian body. This explains their synthesis and mode of action also. A short account of neurohormones and neuroendocrine function is also included.


Chemical messengers, Biochemistry of Hormones & their Feedback Mechanism Zoologist Pakistan 7.4K views • 33 slides

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Chemical messengers DR. Radhakrishna G Pillai Department of Life Sciences University of Calicut

Categories of chemical messengers • Local chemical messengers • Neurotransmitters • Neuropeptides • Hormones • Pheromones

Local messengers • Chemical messengers secreted by certain cells • Alter physiological conditions in the immediate vicinity • Act on same cells (Autocrine) or • Adjacent cells (Paracrine) • Do not accumulate in blood • Eg Lumones: produced in gut-regulate digestion – Histamine : secreted by mast cells in wounds- inflammatory response

Autocrine & paracrine controls

Hormones • Signaling molecules synthesized within the body that – regulate and coordinate physiological and metabolic functions • Act on receptors located on or in target cells • Produced by specialized secretory cells that are either – localized in secretary glands or – within organs that have other primary functions • Depending on the cellular origin and the means by which they reach their targets, the cellular messengers are given different names

Hormones • The original definition of hormones was – restricted to chemical messengers – produced within glands of internal secretion and – released into circulation in small quantities to act on distant receptors • The term hormone is derived from the Greek word hormein: to excite

Hormones • Recent definition of hormones also include; – chemical messengers produced by other than specialized secretary cells and – Signaling molecules that reach their target receptors by routes other than circulation – act on the receptors of the cell that produced them – Such messenger action is called autocrine (Greek :autos meaning self)

Paracrine controls • Chemical messengers acting on the receptors of adjacent cells are called paracrine secretions (Greek: word: para, adjacent) – Eg. somatostatin in the pancreatic islets acting on adjacent insulin and glucagon cells

Chemical messengers and their sources

Non glandular sources of CMs

Chemical messenger: mode of actions

Chemical messenger: mode of actions • Chemical messages are transmitted to specific receptors through – the extracellular fluids – across the synaptic gap – blood • When the chemical messengers stimulate receptors on the cell that synthesized them hormone action is autocrine • Messages delivered to adjacent cells: paracrine and • Circulated through plasma: endocrine

Neurotransmission & neuroendocrine action • Nerve cell deliver chemical messages: – in a paracrine fashion across a synaptic gap represent neurotransmission, and – through the blood, neuroendocrine action

Neuro endocrine system • Endocrine system shares its signaling and coordinating function with the nervous system • The two systems have evolved to control and integrate vital body functions • Chemical messengers produced by nerve cells and released from the axonal endings usually act in endocrine or paracrine fashion • Neural signaling molecules released into the synaptic gap to activate receptors on the adjacent cell membranes are called neurotransmitters

Specificity of neurotransmitters • The specificity of the message is assured both by – The specificity of receptors on the postsynaptic membrane & – The discrete physical alignment between axon terminals of a specific type of neuron and the receptors on the postsynaptic membranse of a particular cell

Neurotransmitters Vs hormones • Neurotransmission is characterized by speed of message transmission (milliseconds) and restricted points of delivery • Hormones act over a longer period of time (seconds to hours) and are distributed diffusely through the extracellular fluid/blood medium to a large number of targets

Examples of neurotransmission > Neurotransmitter acetylcholine released from the axonal endings of the motor nerves into the synaptic gap; Act on the nicotinic cholinergic receptors on skeletal and cardiac muscle fibers

Neurohormones • Neural signalling molecules released into circulation are called neuroendocrine secretions or neurohormones • These messengers behave as hormones by their method of signal transmission • They reach their targets through the circulation • The specificity of receptors on target cells assure the delivery of endocrine message to specific targets – Examples: antidiuretic hormone and oxytocin synthesized by the nerve cells residing in hypothalamus and – released into circulation from their nerve endings in the posterior pituitary gland

Neurotransmission or neurosecretion • Some cells disseminate chemical messages by a combination of endocrine and neural mode of communication – Eg. the sympathetic nerves communicate largely by neurotransmission of norepinephrine (NE) – Some NE is released in to general circulation: act as a neuroendocrine messenger • Cells of adrenal medulla are developmentally and evolutionarily ganglionic neurons of the sympathetic nerves: lost their axons • They release epinephrine (E) and NE into systemic circulation • Their function altered from neurotransmission to neurosecretion

Hormones • Eicosanoids: prostaglandins, thromboxanes, leukotrienes, and prostacyclins are group of signaling molecules included as hormones • They are short-lived chemical messengers • They exert autocrine, paracrine, and occasionally endocrine action on their receptors • NO: only inorganic signaling chemical messenger : control a number of important functions

Hormone structure and synthesis • By chemical structure, hormones fall into three groups: amines, peptides and proteins and lipid derivatives • Amine hormones are metabolites of amino acids – Example: epinephrine, norepinephrine and dopamine • They are collectively called catecholamines • Synthesized from the aromatic amino acids phenylalanine and tyrosine in the brain and in the adrenal medulla • Frequently hormones synthesis is under the control of other hormones • Eg. cortisol controls the last enzyme Phenylethanolamine N- methyltransferase (PNMT ) in the synthesis of epinephrine

Hormone production • Epinephrine is synthesized from the aromatic amino acids tyrosine and phenylalanine • Phenylalanine is an essential amino acid that has to be obtained in the diet • Sympathetic nerves and central nervous system synthesize NE • Adrenal medullary cells produce NE and E • Conversion of NE to E in the medulla is facilitated by the adrenal cortical hormone cortisol

Synthesis of catecholamines

Storage and release of hormones

Storage and release of hormones • Hormones are released mainly in two ways • Amine and peptide hormones are stored in storage vesicles before release • Their secretion is controlled by neurotransmitters, hormones or metabolites • Neural influence impose rhythmicity to hormone secretion • Various secretagogues promote fusion of vesicles and cell membrane • The wall of the vesicles rupture • The hormones are given out by exocytosis into blood or extracellular fluid

Storage and release of hormones • Steroid hormones leave the Golgi apparatus in transport vessicles • Released into circulation by exocytosis without being stored within the cytoplasm • This type of hormone secretion is called constitutive • Here the control of hormonal response is at the stage of hormone synthesis rather than hormone release

Regulated release of hormones • Peptide and amine messengers are stored in secretary vesicles • Their release requires transduction of hormonal, neural, and metabolic stimuli by way of intracellular calcium or second messengers • The release of steroid hormones is constitutive • Here hormonal, neural, and metabolic stimuli control steroid hormone synthesis • Immediately the hormone are released from transport vesicles

Quantity released • Hormones are released in nm to picomolar concentrations • They circulate in the blood at such low concentrations • Success of hormone mediated response transmission rely on the specificity and high affinity of their binding to the receptors • Most hormones are secreted in intermittent rather than continuous fashion • Pulsatile hormone secretion probably serves to supply hormones in quantities required for their action

Frequency of hormone release • The frequency of basal hormone pulses ranges from minutes to hours • There are also seasonal rhythms of hormone release • Various stimuli can acutely increase or decrease hormone secretion • Eg. acute change in secretion of some hormones during exercise • Compensatory changes in hormone secretion usually result from metabolic, neural or hormonal feedback • This feedback control is over the normal hormone secretion signaling: the secretion rate is out of alignment with the acute need for hormone action

Pattern of hormone release and its effect • The temporal pattern of hormone secretion is of remarkable biological significance • The same quantity of hormone can have opposite biological functions when it is delivered in intermittent as opposed to tonic fashion • Eg. lack of growth stimulation by tonic administration of GH • Normal menstrual function by a circhoral (1 pulse/hr) secretion of LH • Lower or higher or tonic release of LH suppress menstrual cycle • Modulation of biological response by changes in the temporal pattern of hormone secretion is often exploited for specific purposes • Tonic administration of gonadotropins has been successfully used to achieve contraception

Transport of hormones • Peptide, amine and protein hormones are water soluble, and most of them circulate freely • All of lipophilic steroid hormones, growth hormone and some growth factors circulate bound to carrier proteins • Albumin and pre albumin non-selectively bind to and transport a variety of small messenger molecules • Globulins have single high-affinity binding sites for specific steroid and amine hormones

Importance of hormone binding to carriers • Binding hormones to carriers extends the period of their availability and action by – Preventing their rapid clearance from plasma – By keeping some of the hormone from circulating in the free form and diminish the magnitude but extend the duration of hormone action – Insulin-like growth factors are protected from degradation and the duration of their biological action extended for hours by the complex system of binding proteins • In some instances (for example GH), binding proteins have almost identical structure to cell receptors and may facilitate the binding of the hormone to the receptor • Thus the binding proteins may facilitate hormone action

Degradation of Hormone • To be most effective, hormones need to reach their target receptors in intermittent fashion. Why? • Receptors become down regulated or less sensitive to hormone action when; – hormones are present in unphysiologically high concentrations or – for abnormally long periods of times • Hormone degradation avoids these situations • Hence it is necessary prerequisite for optimal hormone sensitivity and action

Degradation of hormones • Hormones are changed into metabolicaly inactive form either in target cells or in the liver and kidney • A number of enzymes participate in metabolic degradation of hormones • The speed with which the hormones are metabolized depends on; – whether they travel free in plasma – as well as on their molecular structure and – the characteristics and location of their degrading enzymes • The half-lives of hormones vary from seconds to hours

Half life of some chemical messengers

Thank you pillai_radhakrishna@hotmail.com 0091 9495554891

Editor’s Notes