Fiziologia endocrina

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8 Endocrine physiology Endocrine control involves secretion of a hormone into the circulation. This messenger molecule binds to any cells which carry the relevant receptors, initiating the observed response (Section 1.5). As far as the target cell is concerned this is similar to other forms of chemical signalling, e.g. chemical neurotransmission. Indeed, a given substance may act both as a neurotransmitter and as a hormone at different sites. For example, noradrenaline (norepinephrine) is a pos
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  8.1 Principles of endocrine function207 8.2 Hypothalamic and pituitary function208 8.3 Thyroid function210 8.4 Hormonal control of calcium212 8.5 Functions of the adrenal cortex215 8.6 Function of the adrenal medulla218 8.7 Endocrine functions of the pancreas219Self-assessment:questions223Self-assessment:answers226 8.1Principles of endocrine function Endocrine control involves secretion of a hormone intothe circulation. This messenger molecule binds to anycells which carry the relevant receptors, initiating theobserved response (Section 1.5). As far as the targetcell is concerned this is similar to other forms of chem-ical signalling, e.g. chemical neurotransmission.Indeed, a given substance may act both as a neuro-transmitter and as a hormone at different sites. Forexample, noradrenaline (norepinephrine) is a post-ganglionic sympathetic neurotransmitter but is alsoreleased into the circulation from the adrenal medulla(Section 7.6). Because of the large diffusion distancesand circulation delays involved, however, hormonalresponses are generally slower in onset than thosemediated by nerves. They are also more persistent,since removal of the hormone from the bloodstreammay take some time after secretion has stopped.The release of endocrine substances is controlled inthree ways: Control by a regulated solute. Changes in a con-trolled variable may directly trigger changes in hor-mone levels; e.g. the cells which secrete parathormone,which acts to reduce the concentration of Ca 2+ in theextracellular fluid, are themselves directly sensitive to[Ca 2+ ] (Section 8.4). This is an example of a classicalnegative feedback mechanism. Control by a different hormone. Secretion may beregulated by another hormone, e.g. several pituitaryhormones stimulate secretion from other endocrineglands. This is also often under negative feedbackcontrol, with inhibition of pituitary secretion by theproduct hormone. Positive feedback may occur, how-ever, leading to a rapid rise in hormone levels, e.g.prior to ovulation in the female reproductive cycle(Section 9.4).  Nervous control. Secretion may be under nervouscontrol, and this provides a mechanism for the integra-tion of nervous and endocrine responses. For example,hormone release may be altered by changing sensorystimuli or emotional states. The hypothalamus plays acentral role in linking various endocrine systems withthe nervous system. 207 Endocrine physiology 8 Overview Endocrine regulation is an important element of homeostaticcontrol. Integration of endocrine and nervous controls is amajor function of the hypothalamus. These neuroendocrinelinks will be considered in some detail, along with the actionsof various hormones which regulate cell metabolism or someaspects of fluid and electrolyte balance. Other functions whichare also under endocrine control are considered elsewhere,e.g. cardiovascular performance (Ch. 3), renal function (Ch. 5),gastrointestinal motility and secretion (Ch. 6) and reproductivephysiology (Ch. 9). Learning objectives  At the end of this section you should be able to: q describe the main features of hormonal control q outline how hormone release is controlled.  8.2Hypothalamic and pituitaryfunction The hypothalamus regulates both autonomic nervousactivity (Section 7.7) and several aspects of endocrinefunction. The latter role is fulfilled through its linkswith the pituitary gland, which secretes a wide range ofhormones. Some of these regulate human physiologydirectly, while others control the activity of variousendocrine glands around the body. The result is amultitiered endocrine control system which can beinfluenced through the nervous inputs to thehypothalamus. Relevant structure The hypothalamus is located adjacent to the third ventri-cle in the forebrain and is connected by the hypophysealstalk to the pituitary gland ( hypophysis cerebri ) immediate-ly inferior to it (Fig. 155). This has two, embryologicallydistinct components. q The anterior pituitary , or adenohypophysis , is a classicalendocrine gland containing secretory epithelial cells.It is derived from an outpouching of the developingoral cavity ( Rathke’s pouch ). The anterior pituitaryreceives its blood supply from a series of hypophyseal portal vessels which link it with the hypothalamus. q The  posterior pituitary , or neurohypophysis , develops asa downgrowth from the brain and is continuous withthe hypothalamus itself. It is made up of adaptedaxons which release hormones into the blood fromtheir terminals. The cell bodies of these specializedneurones lie within the  paraventricular and supraoptic nuclei of the hypothalamus. Hypothalamic control of pituitaryfunction This is different for the anterior and posterior lobes ofthe pituitary.  Anterior pituitary  The endocrine activity of the anterior pituitary is regu-lated by hormones secreted from the hypothalamus andtransported in the hypophyseal portal blood. Each ofthese factors selectively promotes or inhibits the secre-tion of a specific pituitary hormone and so they arereferred to as releasing or inhibiting hormones , respective-ly. They are generally oligopeptides, although prolactin-inhibiting hormone is probably the transmittersubstance dopamine. Hypothalamic secretion of thesepituitary-regulating hormones is influenced by neuro-logical inputs and feedback control. Both the pituitaryhormones themselves and circulating products of theendocrine systems controlled by those hormones canregulate hypothalamic secretion, usually through feed- back inhibition (Fig. 156). Posterior pituitary  The posterior pituitary secretes two peptides which aremanufactured in the cell bodies of hypothalamic 208 Endocrine physiology 8 Learning objectives  At the end of this section you should be able to: q outline the development of the pituitary q draw a labelled diagram of the links between hypothalamusand pituitary q explain how pituitary function is controlled and the rolenegative feedback at hypothalamic and pituitary level playsin this q list the anterior and posterior pituitary hormones and outlinetheir actions q describe the main consequences of abnormal pituitaryfunction. Fig.155 The links between the hypothalamus and thepituitary gland.  neurones (Fig. 155). These hormones are packaged invesicles and transported along the axons into theposterior pituitary itself to be stored in the axonterminals. Appropriate sensory stimuli lead to activationof the hypothalamic cells and the release of the hormonesfrom the posterior pituitary into the bloodstream. Thisadaptation of neurotransmission to fulfil an endocrinerole is sometimes called neurocrine control.  Anterior pituitary hormones The anterior pituitary secretes six peptide hormoneswith well-defined functions in humans. These includehormones controlling the endocrine functions of theadrenal cortex, the thyroid and the gonads, as well asprolactin and growth hormone.  Adrenocorticotrophic hormone (ACTH) is secreted inresponse to corticotrophin-releasing hormone (CRH) fromthe hypothalamus. Its action is to stimulate the release ofglucocorticoids from the adrenal cortex. Anumber ofother peptides are synthesized and secreted from theanterior pituitary along with ACTH. These include theendogenous opiate β -endorphin, and precursors of melanocyte-stimulating hormone . Their roles are unclear. Thyroid-stimulating hormone (TSH) , or thyrotrophin,is secreted in response to thyrotrophin-releasing hormone (TRH) from the hypothalamus. It stimulates thyroidsecretion.  Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are secreted in response to  gonadotrophin-releasing hormone (GnRH) from the hypothalamus. Thesegonadotrophins stimulate the male and female gonads(Sections 9.2 and 9.4). Prolactin secretion may be influenced by a  prolactin-releasing hormone (PRH) but is mainly controlled by a  prolactin-inhibiting hormone (PIH), (this is probablydopamine). Stimuli which reduce PIH release from thehypothalamus raise prolactin levels. This occurs duringpregnancy, favouring development of the breasts readyfor lactation, and as part of the suckling reflex during breast feeding. The resultant prolactin peaks stimulatemilk production (Section 9.5).  Human growth hormone (hGH) , or somatotrophin, iscontrolled by two antagonistic hypothalamic hormones. Growth hormone-releasing hormone (GHRH) is believed to be more important than the  growth hormone-inhibitinghormone (GHIH), which is also known as somatostatin.Growth hormone is a major anabolic hormone whichstimulates cell division and growth in both bony andsoft tissues around the body. These effects are probablyindirect, being mediated by growth promoters, knownas somatomedins , produced by the liver in response tohGH stimulation. Growth hormone is particularlyimportant during childhood and adolescence, butalthough it can no longer stimulate long bone growthafter the epiphyses have fused, hGH continues to haveimportant metabolic functions throughout life. Thesefavour protein synthesis in most body tissues, whilstpromoting the breakdown of fat for energy use. This hasa carbohydrate-sparing effect, so that glycogen storesincrease and blood glucose levels tend to rise. Low blood glucose stimulates hypothalamic GHRH andinhibits secretion of somatostatin, both from the hypo-thalamus and the δ cells of the pancreatic islets. Theresulting increase in hGH helps raise glucose concentra-tions back towards normal. Growth hormone secretionis also stimulated by trauma and stress, through neuralcontrol of the hypothalamus. Posterior pituitary hormones The posterior lobe of the pituitary secretes two shortchain peptide hormones, oxytocin and antidiuretichormone. Oxytocin is produced by cells in the paraventricularand supraoptic nuclei of the hypothalamus. These arereflexly activated by sensory inputs from mechano-receptors in the breast during suckling, and oxytocin isreleased from the posterior pituitary in response. Thisstimulates the ejection of milk through contraction of themyoepithelial cells surrounding the milk ducts (Section9.5). Oxytocin also stimulates uterine contraction duringlabour (Section 9.5).  Antidiuretic hormone (ADH) is also produced in theparaventricular and supraoptic nuclei of the hypothal- 209 8 Hypothalamic and pituitary function Fig.156 Summary of the hypothalamic–pituitary controlsystem.Note that secretion of both anterior pituitary andhypothalamic hormones may be inhibited by the circulatinghormones whose release they stimulate.This providesnegative feedback regulation of hormone levels.  amus. Sensory inputs, both from local osmoreceptorsand from stretch receptors in the cardiovascular system(the cardiopulmonary and systemic arterial barorecep-tors), stimulate these cells whenever the osmolality ofthe extracellular fluid rises, or if blood volume or bloodpressure falls. The resulting increase in ADH promoteswater reabsorption from the collecting ducts of the kid-ney (Section 5.4), and so tends to reduce the osmolalityand expand the volume of the extracellular fluids bypromoting the production of a small volume of concen-trated urine. At the same time, peripheral resistance isincreased through arteriolar constriction, which alsohelps to maintain arterial pressure (Section 3.6). Thispressor effect, though less important physiologically,explains the derivation of ADH’s alternative name,which is vasopressin. 8.3Thyroid function The thyroid hormones are key metabolic regulators andare particularly important in determining metabolic rateand heat production. Relevant structure The thyroid gland is located in the neck, in front of and just below the level of the larynx, and consists of twolobes joined by a central isthmus. Histologically, it con-sists of numerous spherical  follicles , each with an outerlayer of cuboidal epithelium and filled with protein-aceous colloid (Fig. 157A). These follicles represent thefunctional subunits of the gland, responsible for synthe-sis, storage and release of the thyroid hormones. Thethyroid also contains  parafollicular C cells , which secretecalcitonin. These will be considered along with otheraspects of body calcium regulation (Section 8.4). Hormone synthesis The first step in the synthesis of the thyroid hormonesinvolves active pumping of iodide ions (I − ) from theextracellular space into the follicular epithelium (Fig.157B). The trapped I − enters the colloid and is oxidized 210 Endocrine physiology 8 after epiphyseal fusion, there is no increase in height,but soft tissue and body organ growth are stimulated.This is known as acromegaly  .A persistent excess ofhGH may eventually lead to elevated blood glucoselevels, i.e.diabetes mellitus results q fluid retention and low plasma osmolality caused byelevated ADH.This is particularly associated withcancer of the lung and is known as inappropriate ADH secretion  .This may be classified into problems arising fromdeficient pituitary secretion and those caused by excesshormone. Deficient secretion Deficiencies resulting from insults to the entire gland,e.g.because of a local tumour or its surgical treatment,may involve all the pituitary hormones, a conditionknown as panhypopituitarism  .Alternatively, there may bea deficit in a single hormone caused by a defect in therelevant pituitary cells or in the hypothalamic cells whichregulate their function.The main problems which canresult are: q corticosteroid deficiency caused by lack of ACTH(Section 8.5) q thyroid deficiency caused by lack of TSH q failure of sexual function caused by lack of FSH/LH q dwarfism caused by lack of hGH in childhood;thismay arise as a congenital defect and can be treatedwith hGH supplements q diabetes insipidus caused by lack of ADH;this resultsin an inability to concentrate urine and so the patientpasses 8–10L of dilute urine per day and has to drinka commensurate volume. Excess secretion The following conditions resulting from over secretion ofa single pituitary hormone are observed in clinicalpractice: q corticosteroid excess caused by elevated ACTH(Section 8.5) q impaired reproductive function caused by elevatedprolactin. Hyperprolactinaemia  is now recognized asan important cause of menstrual failure and infertility.This seems to be caused by prolactin’s ability to blockGnRH production by the hypothalamus q abnormal growth caused by elevated hGH.Thisstimulates long bone growth in children, causing gigantism  .If hGH levels become raised in adult life, Box 32 Clinical note:Abnormal pituitary function Learning objectives  At the end of this section you should be able to: q name the main cell types in the thyroid q outline how thyroid hormones are synthesized q explain how thyroid secretion is controlled q explain the significance of protein transport of thyroidhormones q list the main actions of thyroid hormones q describe the main consequences of abnormal thyroidfunction.
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