Physiology of the pituitary, thyroid, parathyroid and adrenal glands

BASIC SCIENCE

Physiology of the pituitary, thyroid, parathyroid and adrenal glands

system. The utility of this vascular system is that minute quantities of hypothalamic hormones are carried directly to their target cells in the anterior pituitary, and are not diluted in the systemic circulation. Growth hormone (GH) GH is a 191-amino acid single-chain polypeptide synthesized in somatotroph cells of the anterior pituitary. There are about ten pulses of GH secretion per day. The predominant male ‘pulsatile’ secretion versus female ‘continuous’ secretion might explain the different patterns of gene activation in target tissues, for example induction of linear growth patterns and gain of body weight. GH secretion is controlled as follows.
GH-releasing hormone (GHRH) secreted by the hypothalamus or as an ectopic secretion (e.g. from pancreatic cancers) stimulates GH secretion.
Somatostatin (SST) inhibits GH secretion. In addition SST has multiple effects on pancreatic, liver and gastrointestinal function. It inhibits the secretion of CCK, glucagon, gastrin, secretin, GIP, insulin and vasoactive intestinal peptide (VIP) from the pancreas.
Glucocorticoids have a biphasic effect on GH secretion: an initial acute stimulation within 3 hours, followed by suppression within 12 hours.
Catecholamines: a-adrenergic pathways stimulate GH secretion. The a2-agonist clonidine can therefore be used as a provocative test of GH secretion. b-adrenergic pathways inhibit secretion by increasing somatostatin release.
Acetylcholine: muscarinic pathways stimulate GH secretion by modulating somatostatinergic tone. Pyridostigmine, an indirect agonist which blocks acetylcholinesterase, increases the 24-hour secretion of GH. On the other hand, atropine (muscarinic antagonist) blunts GH release.
Endogenous opioids: endorphins and enkephalins stimulate GH secretion in man and blockade with opiate antagonists can attenuate the GH response to exercise.
Exercise is a powerful stimulus to secretion of GH.
Hypovolaemic shock, elective surgery, hypo- and hyperglycaemia, and malnutrition all cause increased GH release. On the other hand, obesity is associated with lower GH levels, partially due to decreased frequency of GH pulses.
GH release is stimulated by a protein meal. L-arginine, an essential amino acid, can be used as a provocative test for GH secretion.
Sleep: the amount of GH secreted during sleep is approximately triple the daytime rate. The decline in GH secretion during ageing is parallelled by the decreasing proportion of time spent in sleep. After sleep deprivation (e.g. experimental or due to ‘jet lag’ when travelling across many time zones) the magnitude of secretory spikes is augmented and the major pulse of GH secretion occurs in late sleep.

Radu Mihai

Abstract The pituitary gland is made of clusters of cells producing specific hormones that control growth (growth hormone), thyroid function (triiodothyronine (T3) and thyroxine (T4)), adrenal function (adrenocorticotrophic hormone (ACTH)) and gonadal function (follicle-stimulating hormone and luteinizing hormone). In addition, the neurons that join the posterior pituitary (neurohypophysis) secrete vasopressin e the antidiuretic hormone involved in maintaining water balance. The negative feedback loop is the basic mechanism to control the regulation of all endocrine glands. Hypothalamic peptides e releasing hormones (e.g. TRH, corticotrophin-releasing hormone) reach the hypophysis via the portal venous system and induce the secretion of specific stimulating hormones (e.g. thyroid-stimulating hormone, ACTH) that drive the end-target endocrine cells to secrete hormones (e.g. thyroid hormones e T3 and T4 or adrenal hormones e cortisol, dehydroepiandrosterone sulphate). The plasma levels of these circulating hormones inhibit the pituitary (short feedback) or the hypothalamus (long feedback) and limit the further release of releasing and stimulating hormones. The effects of circulating hormones on different tissues are mediated via specific receptors on the cell membrane (e.g. vasopressin receptors), in the cytoplasm (steroid receptor for cortisol) or in the nucleus (e.g. thyroid hormone receptors). Understanding the physiological effects of peripheral hormones helps understanding the mechanisms by which clinical signs and symptoms develop in diseases characterized by excessive hormone secretion (e.g. thyrotoxicosis, Cushing syndrome, phaeochromocytomas) or lack of hormone secretion (e.g. diabetes insipidus). The parathyroid gland and adrenal medulla are not controlled by the pituitary but play important roles in calcium metabolism and the adrenergic (sympathetic nervous system) function respectively.

Keywords Catecholamines; cortisol; hormone secretion regulation; physiology; pituitary; thyroid hormones

Pituitary gland The pituitary gland (hypophysis) lies beneath the hypothalamus, in the sella turcica and is composed of two parts (Figure 1): the anterior pituitary (adenohypophysis) is derived from ectoderm and secretes protein hormones; the posterior pituitary (neurohypophysis) is composed largely of hypothalamic neuronal axons which also form the pituitary stalk. Secretion of hormones from the anterior pituitary is controlled by hypothalamic hormones reaching the pituitary via a portal

Adrenocorticotrophic hormone (ACTH) ACTH is released from corticotrophs. ACTH is derived from a larger amino acid precursor, pro-opiomelanocortin (POMC). POMC transcription is positively regulated by corticotrophinreleasing hormone (CRH) and negatively regulated by glucocorticoids. Like GH, ACTH is secreted in pulses from corticotrophs

Radu Mihai FRCS is a Consultant Endocrine Surgeon and Honorary Senior Clinical Lecturer in the Department of Endocrine Surgery at John Radcliffe Hospital, Oxford, UK. Conflicts of interest: none declared.

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Pituitary gland: anatomical connections and functional role

Paraventricular nucleus

Preoptic nuclei Gonadotropin FSH

Dorsomedial nucleus Arcuate nucleus

Supraoptic nucleus

Ventromedial nucleus Hypothalamus Och

Median eminence Gonadotropin FSH

Anterior Posterior Pituitary lobe

FSH, follicle-stimulating hormone. Figure 1

Antidiuretic hormone (ADH)/vasopressin (AVP) Arginine-vasopressin is a nine-amino acid peptide synthesized within hypothalamic neurons and packaged in secretory vesicles with a carrier protein called neurophysin, to be released from the posterior pituitary. Vasopressin conserves body water by reducing the loss of water in urine. It binds to receptors on cells in the collecting ducts of the kidney and promotes the insertion of ‘water channels’ (aquaporins) into the membranes of kidney tubules, which transport solute-free water through tubular cells and back into blood (water reabsorbtion), leading to a decrease in plasma osmolarity and an increased osmolarity of urine. High concentrations of ADH also cause widespread constriction of arterioles, which leads to increased arterial pressure. ADH secretion is modulated by plasma osmolarity, which is sensed in the hypothalamus by osmoreceptors. When plasma osmolarity increases above a threshold, osmoreceptors stimulate the neurons that secrete ADH. Hypothalamic osmoreceptors also control the thirst sensation. The osmotic threshold for ADH secretion is considerably lower than for thirst, hence thirst is only activated if ADH alone cannot handle the increase in osmolarity. Secretion of ADH is also simulated by decreases in blood pressure and volume, conditions sensed by stretch receptors in the heart and large arteries. For example, loss of 15e20% of blood volume by haemorrhage results in massive secretion of ADH.

with about 40 pulses/24 hours, correlating with the pulsed secretion of cortisol. ACTH levels vary in circadian rhythm, with a peak at 0600e0900 hours and a trough at 2300e0200 hours. Glucocorticoid feedback occurs at multiple levels: at the pituitary (inhibition of POMC transcription), at the hypothalamus (inhibition of CRH and AVP synthesis and release in the PVN), and most importantly, centrally at the level of the hippocampus, which contains the highest concentration of glucocorticoid receptors in the central nervous system. ACTH release is increased by several factors, such as follows.
CRH is a neuropeptide mainly found in the paraventricular nuclei of the hypothalamus. Besides stimulating POMC transcription and ACTH synthesis, CRH stimulates the release of ACTH, leading to a biphasic response with the fast release of a pre-synthesized pool of ACTH, and the slower and sustained release of newly synthesized ACTH.
VIP stimulates ACTH secretion, a mechanism which may explain the increase in ACTH after eating.
Catecholamines stimulate CRH release via central a1adrenergic receptors.
Interleukins (IL-1, IL-6 and possibly IL-2) e via short-term effects on the hypothalamus.
Stress induces the release of ACTH. The hypoglycaemia during the insulin tolerance test is one such stressor.

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Diabetes insipidus (DI) is due to a lack of ADH biological activity: hypothalamic (‘central’) DI results from a deficiency in secretion of ADH from the posterior pituitary (e.g. after head trauma or infections or tumours involving the hypothalamus); nephrogenic DI occurs when the kidneys are unable to respond to ADH (e.g. renal disease or mutations in the ADH receptor gene or gene encoding aquaporin-2). The major sign of either type of diabetes insipidus is excessive urine production. If AVP is completely absent, humans produce as much as 16 litres of urine per day! Hypothalamic DI can be treated with exogenous vasopressin (DDAVP).

concentration of iodine some 40 times higher than circulating levels, hence the thyroid contains more than 90% of total body iodine. Several other ions (pertechnetate, perchlorate) can be transported by NIS. This forms the basis for the use of technetium (99mTc) for thyroid scintigraphy. The primary regulation of NIS is through thyroid-stimulating hormone (TSH) and circulating iodine levels. Rapid increase in iodine levels leads to a shut-down of iodine incorporation (WolffeChaikoff effect), which is a protective mechanism against iodine overload. This effect is beneficial in patients who need rapid blockade of thyroid gland activity either for therapeutic reasons (e.g. patients with Graves’ disease who develop allergic reaction to medication and need urgent thyroidectomy) or prophylactic (e.g. after the fallout of radioactive iodine after a nuclear accident such as the Chernobyl disaster in 1984). Before administration of radioactive tracers marked with I131 or I123, high levels of TSH are critical for an efficient therapeutic administration of radioactive iodine in patients with thyroid cancers. This is achieved by redrawing the liothyronine (T3) replacement therapy some 10e14 days before the administration of I131 (to induce TSH secretion from the pituitary) or by injecting human recombinant TSH (Thyrogen). Loss of NIS expression occurs in poorly differentiated thyroid cancers. Such patients do not benefit from radioactive iodine treatment. There are ongoing efforts to identify drugs that could induce NIS expression in such tumours with the hope of reestablishing their ability to concentrate I131. Within the follicular cells the iodine is oxidized to iodide and transported into the follicular lumen by an iodineechloride transporter e pendrin. Mutations in the pendrin gene are associated with congenital goitre and deafness.

Thyroid gland The developing thyroid bud appears at the base of the tongue (foramen caecum) and it migrates caudally from the pharyngeal floor, passes through or in front of the hyoid bone and it reaches its final position in front of the trachea. Along this migration path, a connection between the foramen caecum and the thyroid gland can persist as a thyroglossal duct. Parafollicular C-cells derive from the neuroectoderm at the level of the fourth pharyngeal pouch, merge with the developing thyroid and end-up concentrated predominantly on the posterior side of upper third of each thyroid lobe. Thyroid follicles represent the dominant histological feature of the mature thyroid gland (Figure 2). Thyroid cells surround a central follicular lumen filled with a clear proteinaceous colloid. The apical surface of the cells line the follicular lumen and the basal part of each cell rests on a thin basement membrane that isolates them from surrounding capillaries. Calcitonin producing C-cells are identified easily in patients with multiple endocrine neoplasia (MEN-2) presenting with C-cell hyperplasia (Figure 2).

Thyroglobulin (TG) TG is a glycoprotein synthesized only by follicular cells and acts as the storage site for thyroid hormones within the colloid. Small amounts of colloid are engulfed through pinocytosis into vesicles that are transported inside the follicular cells. Lysosomes then fuse with these vesicles and release T4/T3. Each TG molecule stores ten times more T4 than T3. During the process of generating T4/T3, tyrosine residues on the TG molecule are coupled with iodine. This iodination process is called organification and is mediated by the enzyme thyroid peroxidase (TPO). As a result, monoiodotyrosine (MIT) and diiodotyrosine (DIT) are formed. Subsequently TPO mediates the coupling of MIT and DIT (forming active T3 or the inactive form e reverse T3 hormone) or two DIT molecules to form T4. Antithyroid drugs (carbimazole and propylthiouracil) inhibit the enzymes involved in the synthesis of thyroid hormones. Plasma TG levels are measured during follow-up after treatment for thyroid cancer. In response to total thyroidectomy plus radioactive iodine ablation it is expected that all thyroid tissue would be destroyed hence TG would drop to undetectable levels and would rise only in the presence of recurrent/metastatic disease.

Iodine Production of thyroid hormones is critically dependent on iodine, deficiency of which can lead to endemic goitre, hypothyroidism or cretinism (in children whose mothers had severe hypothyroidism) and favour the development of follicular thyroid cancer. Conversely, excess iodine intake is associated with autoimmune thyroid disease and papillary thyroid cancer. Iodine is absorbed very efficiently in the gastrointestinal tract, reaches the systemic circulation and is concentrated in the follicular cells by a plasma membrane protein, the sodium-iodine symporter (NIS). NIS activity allows for creating an intracellular

Histological appearance of the thyroid gland

colloid

Circulating thyroid hormones Over 99% of circulating T4 and T3 are bound to plasma proteins: thyroid-binding globulin (TBG, 75%), thyroid-binding prealbumin (TBPA, 15%), and albumin (10%). Only 0.02% of T4 and 0.4% of T3 are free in the circulation. As a greater percentage of T4 is bound

Figure 2 Low and high power view of thyroid follicles.

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the half-life of T4 is longer (approximately 7 days) compared with T3 (approximately 12e24 hours). Because pregnancy and contraceptive pills can increase the synthesis of binding proteins, such patients can have a higher level of total T4/T3 but normal free T4(fT4)/free T3(fT3). This effect also explains why pregnant women need an increased dose of thyroxine substitution to maintain normal TSH levels. Thyroid hormones can be displaced from its binding sites by aspirin and non-steroidal anti-inflammatory drugs (NSAIDs). Such drugs should be avoided in patients with severe thyrotoxicosis as they can increase the level of free hormones hence worsening the clinical signs. Routine blood tests for thyroid functions tests (TFTs) include measurement of TSH and fT4 levels. In primary hyperthyroidism (e.g. Graves’ disease, toxic multinodular goitre) fT4 is raised and TSH inhibited. In hypothyroidism (e.g. autoimmune thyroiditis, post-thyroidectomy in the absence of T4 replacement) TSH is raised in response low fT4/fT3.

regulating synaptogenesis, neuronal integration, myelination and cell migration. Metabolic effects of thyroid hormones include regulation of basal metabolic rate and increasing oxygen consumption in most target tissues. In addition, thyroid hormones increase the sensitivity of target tissues to catecholamines, thereby increasing lipolysis, glycogenolysis, and gluconeogenesis. Physiology of thyroid crisis: a sudden rise in the level of thyroid hormones leads to thyrotoxic crisis. Seldom seen, it can be triggered by operating on thyrotoxic patients poorly controlled by medication. Signs include severe tachycardia, pyrexia and neurological signs including coma. Treatment involves blocking thyroid secretion (by high-dose oral iodine, see above) and inhibiting the synthesis of thyroid hormones (by using carbimazole or propylthiouracil), decreasing the peripheral conversion of T4 into T3 (propranolol, steroids) and by avoiding displacement of thyroid hormones from binding proteins (i.e. avoid the use of aspirin or NSAIDs).

Control of thyroid function by TSH Thyroid-stimulating hormone (TSH) is a glycoprotein secreted by pituitary cells through a feedback loop with T4/T3 levels (Figure 3). The control cascade starts with TRH e produced in the hypothalamus and released through the hypothalamicpituitary circulation in the pituitary. TRH then stimulates the thyrotroph cells to produce TSH. TSH acts on specific receptors on the membrane of follicular cells and stimulates the activity of NIS (i.e. stimulates iodine uptake) and of intracellular enzymes involved in thyroid hormone synthesis (i.e. stimulates synthesis of TG iodination of tyrosine resides on TG). Mutations in TSH receptor structure lead to autonomous stimulation (toxic adenoma, Plummer’s adenoma). On I123 scan the overactive nodule appears ‘hot’ as it is able to incorporate iodine without the need of circulating TSH while the rest of the gland appears ‘cold’ (i.e. does not incorporate I* as NIS activity in normal follicular cells is absent if TSH is suppressed) (Figure 4). Activating autoantibodies against the TSH receptors are present in Graves’ disease.

Calcitonin Calcitonin belongs to a larger family called calcitonin generelated peptides (CGRP) that are secreted in the brain and gastrointestinal tract. High calcium levels stimulate calcitonin release and this leads to homeostatic inhibition of osteoclast activity (i.e. less bone resorption). This physiological mechanism explains the rationale for using calcitonin in the treatment of osteoporosis and Paget’s disease (though the clinical efficacy is minimal). The role of calcitonin in human biology remains vague. There are no known side effects of low calcitonin levels and very high levels (e.g. in patients with medullary thyroid carcinoma) have no impact on calcium homeostasis.

Parathyroid glands

Conversion of T4 into T3 The enzyme 50 -deiodinase converts T4 into the metabolically active T3 in liver/muscle (type I deiodinase) and brain (type II deiodinase). Type III deiodinase inactivates thyroid hormones. The balance between the activity of each of these enzymes is disturbed in stress. All these enzymes require selenium for their activity hence lack of selenium leads to abnormal thyroid function.

Parathyroid glands begin to develop in the fifth to sixth gestational week from the endoderm of the dorsal parts of third and fourth pharyngeal pouches. From week 7, the parathyroids begin to migrate caudally and medially with the thyroid and thymus. The inferior glands, derived from the third pharyngeal pouch (hence their name P-III) travel the furthest and about 50% tend to be located on the dorsal surface of the thyroid capsule. Alternatively, inferior glands migrate further into the thyrothymic ligament or thymus gland. The superior parathyroid glands derive from the fourth pharyngeal pouch (P-IV) and they attach to the thyroid primordium, reaching a higher and more constant location than the inferior parathyroids.

Cellular effects of thyroid hormones Plasma membranes of various cells have unique hormone transporters for uptake of thyroid hormones. Within the cells, T3 translocates into the nucleus, binds to nuclear receptors (TR, see Figure 3) then couples to thyroid responsive elements (TRE, see Figure 3) on the promoters of target-genes. This process can enhance or inhibit the expression of target genes. Thyroid hormones play an important role in development: they are critical for normal development of the skeletal system and musculature and are essential for normal brain development,

Parathyroid physiology: calcium homeostasis is a phylogenetically important adaptation mechanism involving a balance between intestinal absorbtion, renal excretion and bone mineral deposition modulated by the balance between parathyroid hormones, vitamin D and (possibly) calcitonin. Extracellular calcium concentration ([Ca2þ]) is the main factor controlling the function of parathyroid cells. The calcium sensing receptor (CaSR) expressed on the plasma membrane of parathyroid cells enables them to detect and respond to minute changes in [Ca2þ]. The CaSR receptor is a G-protein coupled

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neonatal hyperparathyroidism. Activating mutations are rare and lead to the reverse condition: hypercalciuric hypocalcaemia. Pharmacological stimulation of the CaSR with a specific agonist (Cinacalcet) inhibits parathyroid secretion and is increasingly used in patients with secondary hyperparathyroidism. Parathyroid hormone e PTH is an 84-amino acid peptide with a short half-life of about 2e5 minutes because of rapid degradation into an amino-terminal (1e34, with biological activity) and a carboxy-terminal fragment. Modern laboratories use a double antibody immunoassay that detects only the intact molecule. The main physiological effects of PTH (Table 1) are related to action on the receptors located in the proximal and distal renal tubular cells and on osteoblasts. In addition, chondrocytes, vascular smooth muscle cells, fat cells, brain and pancreas express PTH receptors.

Regulation of thyroid function and intracellular effects of thyroid hormones Tg (MIT, DIT, T3 and T4 ) H2O2 Iodide

Colloid

Tg E

NIS gene Pendrin, AIT

TPO DUOX

Nucleus

A Transcription B

mRNA mRNA Protein synthesis

NIS C

TSH-R

Adrenal glands

D T3 and T4

Iodide

The adrenal glands are located on the medial side of the upper pole of each kidney and are formed by two areas e the cortex and the medulla, which have different embryological origins and physiological functions. The adrenal cortex represents around 85% of the adrenal gland weight and surrounds the adrenal medulla (Figure 5). The steroid hormones of the adrenal cortex are derivatives of cholesterol. Based on the total number of carbon atoms, the principal types of hormones are either C21 (e.g. cortisol, aldosterone) or C19 (androgens).

Figure 3 TSH signalling via the TSH receptor (which is shown at the bottom of the thyrocyte on the left) controls thyroid hormone synthesis, and it can increase expression of NIS in the basolateral membrane of thyrocytes. The proteins involved in efflux of iodide at the apical membrane are not known, and the roles of AIT and pendrin are unclear. As shown in the left-hand thyrocyte, iodide is organified in the tyrosyl residues of Tg in a reaction catalysed by TPO, in the presence of H2O2, which is produced by DUOX. Tg contains MIT, DIT, T3, and T4 and is stored in colloid until T3 and T4 need to be released into the blood. AIT, apical iodine transporter; DIT, di-iodotyrosine; DUOX, dual oxidase; MIT, monoiodotyrosine; NIS, sodium-iodide symporter; Tg, thyroglobulin; TPO, thyroid peroxidase; TSH-R, TSH receptor; mRNA, messenger RNA.

Glucocorticoids The hypothalamicepituitary axis (HPA) regulates the adrenal release of glucocorticoids. Hypothalamic release of corticotropinreleasing hormone (CRH) stimulates the pituitary gland to

receptor which activates a cascade of events (including activation of phospholipase C and inositol triphosphate 3) and results in release of calcium from intracellular calcium stores into the cytoplasm. It remains difficult to explain why/how high intracellular calcium inhibits PTH secretion while in other endocrine cells it stimulates secretion by promoting the docking and fusion of the secretory granules with the plasma membrane. Abnormalities of CaSR have significant impact on parathyroid physiology. Inactivating mutations lead to hypocalciuric hypercalcaemia (as the cells are unable to be ‘switched off’ in the presence of high [Ca2þ]) in heterozygous patients while newborns homozygous for this mutation (CaSR /) have severe

Physiological effects of parathyroid hormone Tissue

Physiological effects

Renal tubules

C

C

C

Thyroid uptake scan

Figure 4 Intense uptake of technetium into a ‘hot’ nodule. Note that the rest of the thyroid is not apparent as no uptake can occur in the normal thyroid tissue in the absence of thyroid-stimulating hormone.

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Bone

C

Intestine

C

Effects in primary hyperparathyroidism

Reduces renal excretion of calcium Increases excretion of phosphate, Na, K and bicarbonate Increases the activity of 1 a-hydroxylase, enabling 1a hydroxylation (and activation) of 25(OH) vitamin D Inhibits osteoblasts and stimulates osteoclast-mediated bone resorption Increases calcium absorption indirectly by increased activation of vitamin D in the kidney

Hypercalciuria Hypophosphataemia Hyperchloraemic acidosis Renal tubular acidosis

Increase alkaline phosphatase activity and excretion of hydroxyproline

Table 1

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Structure of the adrenal cortex morphological structure (a) and biochemical pathways for steroid hormones synthesis (b) a Hormone secreted

Factors acting on the gland

Zona glomerulosa Mineralocorticoids (aldosterone)

Angiotensin and corticotropin (ACTH) Capillaries Zona fasciculata

Glucocorticoids (cortisol and corticosterone) Corticotropin Androgens (dihydroepiandrosterone androstenedione)

Zona reticularis Glucocorticoids Corticotropin Androgens

b

Steroid hormone synthesis pathways Adrenal glands Cholesterol Pregnenolone Aldosterone

Progesterone Cortisol

Kidneys

Liver

Aldosterone

Cortisone

DHEA Androstenedione

Estrone Testosterone Estradiol Androstenedione Estriol Progesterone Androstenedione Testes Ovaries

Normal pathway

Adrenal fati gue / Pregnenolone steal

ACTH, adrenocorticotrophic hormone; DHEA, dehydroepiandrosterone.

Figure 5 SURGERY 32:10

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Mechanisms of action of cortisol Cortisol

hsp90

GR

Cytoplasmic activation Active GR monomer

FKBP52 hsp90 hsp90

GR

hsp70

FKBP52 hsp90

GR hsp70

hsp90 FKBP52

Dimerization

GR hsp90

hsp70 GR Transcription

GR

GR

GRE GR, glucocorticoid receptor; GRE, glucocorticoid responsive element; hsp, heat shock protein. Figure 6

produce ACTH, which in turn acts on the adrenal cortex to stimulate the synthesis and release of cortisol. Cortisol then completes the cycle by exerting negative feedback on CRH and ACTH release. This feedback mechanism is lost in patients with Cushing’s syndrome (i.e. with an autonomous unilateral adrenal adenoma producing excess cortisol) in whom large doses of dexamethasone fail to suppress the endogenous production of cortisol. In contrast, patients with Cushing’s disease (pituitary adenoma producing ACTH) respond to the inhibition by high-dose dexamethasone (8 mg/day for 2 days) by decreasing cortisol levels.

Action of glucocorticoids (Figure 6): glucocorticoids diffuse passively across the cellular membrane and bind to the intracellular glucocorticoid receptor (GR) expressed in almost every cell in the body. GR regulates genes controlling the development, metabolism and immune response. Because the GR gene is expressed in several forms, it has many different effects in different parts of the body. In the absence of cortisol, the glucocorticoid receptor (GR) resides in the cytosol complexed with a variety of proteins including heat shock proteins (hsp90, hsp70). After the receptor binds to glucocorticoid, the receptor-glucocorticoid complex has two principal mechanisms of action:
Transactivation: a direct mechanism of action involving homodimerization of the receptor, translocation via active transport into the nucleus, and binding to specific DNA sequences (glucocorticoid responsive elements, or GREs). This results in either enhancement or suppression of transcription of susceptible downstream genes with the biological response dependent on the cell type.
Transrepression: activated GR binds with other transcription factors and prevents them from binding to their target genes and hence represses the expression of genes that are

HPA axis testing: to assess whether the HPA axis is functional (e.g. after long-term steroid therapy) cortisol secretion is stimulated with synthetic ACTH (cosyntropin 250 mg) and serum cortisol levels are measured 30e60 minutes later. Normal response is defined as a serum cortisol of at least 600. Glucocorticoid levels have a diurnal variation with a morning peak (0400e0800 hours) and minimal nocturnal trough (0200e0400 hours). Synthesis of cortisol can increase five- to tenfold under conditions of severe stress. A loss of this circadian rhythmicity is seen in patients with Cushing’s syndrome.

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normally upregulated by nuclear factor kB (NF-kB) or activator protein-1 (AP-1) (e.g. pro-inflammatory mediators).

expanded intravascular volume and suppresses renin secretion. Part of the urinary Naþ absorption is in exchange for Kþ and Hþ excretion. In primary hyperaldosteronism (Conns’ syndrome), excess aldosterone leads to high blood pressure, suppressed renin levels, increased urinary extraction of Kþ (leading to hypokalaemia) and Hþ (i.e. acidic urine).

Anti-inflammatory effects: glucocorticoids have inhibitory effects on a broad range of specific immune responses mediated by T cells and B cells, as well as potent suppressive effects on the functions of phagocytes. Through their inhibitory effects on both acquired and innate immunologic function, glucocorticoids are remarkably efficacious in ameliorating many of the acute manifestations of inflammatory and autoimmune disorders. Side effects of glucocorticoids are mostly seen with oral and injectable glucocorticoids, but can be seen with inhaled and topical steroids at higher doses. Glucocorticoid toxicity is related to both the average dose and cumulative duration of use:
Osteoporosis: is one of the most debilitating complications of glucocorticoid therapy. Several mechanisms are involved: decline in bone formation (through a direct inhibition and apoptosis of osteoblasts), increase in bone resorption, a decrease in gastrointestinal absorption of calcium, increase in urinary calcium excretion, and a decrease in gonadal steroid production. Bone loss can be rapid and substantial: as much as 25e30% of bone loss can occur in the spine during the first year of therapy. A substantial increase in fracture risk can occur within 3e6 months of steroid treatment. If steroids are discontinued, bone improves substantially after 6e24 months.
Hyperglycaemia: glucocorticoids increase hepatic glucose production (in part by increasing substrate availability through proteolysis and lipolysis), induce insulin resistance and hyperinsulinaemia and inhibit glucose transport into cells.
Hypertension: glucocorticoids raise blood pressure in both normotensive and hypertensive patients. The pathogenesis is multi-factorial, involving increased peripheral vascular sensitivity to adrenergic agonists, increased hepatic production of angiotensinogen (renin substrate), and activation of renal mineralocorticoid receptors.

Rationale for Conn’s diagnosis: primary hyperaldosteronism is an uncommon cause of hypertension. Excess secretion of aldosterone from a small adrenal adenoma or from bilateral hyperplasia leads to hypokalaemia and hypertension. Screening for this condition in a hypertensive patient relies on measuring the aldosterone/renin ratio (high in primary hyperaldosteronism, low in other forms of hypertension). Adrenal androgens At the onset of puberty, the adrenal starts to secrete weak androgens that trigger the appearance of pubic and axillary hair. During adulthood adrenal androgens have minimal contribution (testicles produce much larger quantities). During menopause the adrenal is an important source of oestrogens. Of historical interest only,

Adrenergic receptors and their physiological effects Tissue a1 Smooth muscle C Blood vessels C Bronchi C Bladder C Iris (radial muscle) C Cardiac Liver a2 Blood vessels Pancreatic b-cells Sympathetic nerve endings b1 Heart

The mineralocorticoids Aldosterone is synthesized and released from zona glomerulosa of the adrenal cortex. Aldosterone secretion is regulated by the renin angiotensin system and is independent of ACTH. Therefore patients with secondary adrenal insufficiency due to previous glucocorticoid therapy have intact aldosterone secretion. In response to a decrease in intravascular volume, renin is released from the juxtaglomerular cells located in the wall of the afferent glomerular arterioles. Renin cleaves angiotensinogen into angiotensin I, which is further converted into angiotensin II by the angiotensin-converting enzyme (ACE). Angiotensin II has two effects: it acts as a potent vasoconstrictor and stimulates the release of aldosterone by binding to plasma membrane receptors on adrenal cells in the zona glomerulosa. In addition to the renin angiotensin system, high Kþ increases aldosterone levels and severe Naþ depletion stimulates the conversion of corticosterone into aldosterone. Aldosterone binds to minerolocorticoid receptors in the distal tubules and cortical collecting tubes of the kidney. It increases sodium absorption through the Naþ channels. This leads to

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Juxtaglomerular cells Sympathetic nerve endings b2 Smooth muscle C Blood vessels C Bronchi C Bladder Heart Sympathetic nerve endings Pancreatic b-cells Skeletal muscle b3 Fat Subcutaneous tissue

Action

Sensitivity NA ¼ ADR

Vasoconstriction Bronchoconstriction Contraction Contraction (mydriasis) Contraction Glycogenolysis Vasoconstriction Decreased insulin secretion Decreased NA release Increased contraction Tachycardia Increased renin secretion Increased NA release

ADR > NA

NA ¼ ADR

ADR >> NA Vasodilatation Bronchodilatation Relaxation Increased rte/contraction Increased NA release Increased insulin secretion Tremor Thermogenesis Lipolysis

NA >> ADR

ADR, adrenaline; NA, noradrenaline.

Table 2

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adrenalectomy was once performed as part of the treatment for advanced breast cancer, in an attempt to reduce circulating oestrogens that could potentially stimulate tumour growth.

phaeochromocytomas (when labelled with I131, which has a longer half-life). Synthesis of catecholamines In the cytoplasm, tyrosine hydroxylase converts tyrosine into dihydroxyphenylalanine (DOPA) and DOPA decarboxylase generates dopamine. Dopamine is actively incorporated into secretory granules and is then converted into norepinephrine. Some chromaffin cells convert norepinephrine into epinephrine if they express the enzyme PNMT (phenyl-ethanolamine-N-methyltransferase). PNMT expression is induced by cortisol, whose high concentration in the adrenal medullary sinusoids drives the production of epinephrine in the normal adrenal gland. Lack of cortisol-induced PNMT expression explains why extra-adrenal phaeochromocytomas (i.e. paragangliomas) exclusively produce norepinephrine. Secretory granules also contain chromogranin, neuropeptide Y, encephalins and somatostatin; though their physiological significance is unclear. Plasma levels of chromogranins are monitored in patients with neuroendocrine tumours as a diagnostic test and a marker of response after therapy.

Basics of congenital adrenal hyperplasia (CAH): CAH is a genetic condition leading to a deficit of one of the enzymes involved in cortisol synthesis. The persistently low cortisol stimulates ACTH secretion and this drives the adrenal hyperplasia. The precursors of cortisol accumulate upstream of the deficient enzyme and are diverted towards production of androgens. The most common enzyme found to be defective is 21hydroxylase. In the female fetus, it leads to variable degrees of virilization of external genitalia, possibly with complete closure of labia (that can be confused with a scrotum) and enlargement of clitoris (pseudohermaphroditism). The young girl born with ambiguous genitalia may therefore be declared a boy and the diagnosis be delayed for many years. In a minority of cases, the enzymatic block is extremely severe and impairs the production of aldosterone leading to severe salt-losing state. Treatment of this condition consists of steroid replacement.

Degradation of catecholamines COMT (catechol O-methyltransferase) generates metanephrines by converting noradrenaline into normetadrenaline and adrenaline into metadrenaline. This is a constant process that takes place both in normal and tumour cells. Measurement of metanephrines in a 24-hour urine specimen is an accurate test for diagnosing a phaeochromocytoma. The end product of catecholamine metabolism is VMA (vanilylmandelic acid) whose urine concentration is now an outdated test for diagnosis of phaeochromocytomas. Only a very small amount of catecholamines are secreted in the urine as free catecholamines (dopamine, adrenaline and noradrenaline).

Adrenal medulla Adrenal medulla develops from the neuroectoderm. In the second month of gestation, cells from the neural crest (sympatogonia) migrate to form the sympathetic system (neuroblasts) or the adrenal medulla primordium (phaeochromoblasts). Similar cells are present in extra-adrenal tissues, predominantly around the aorta (e.g. organ of Zuckerkandl at the bifurcation of the aorta) but also in the neck and rarely in the bladder. Such cells can give rise to extra-adrenal phaeochromocytomas, called paragangliomas. One characteristic in common with neurons is the expression of the norepinephrine uptake mechanism. This uptake mechanism is responsible for incorporation of MIBG (meta-iodinebenzylguanetidine) into chromaffin cells. This compound can be marked with radioactive iodine and used as a radiopharmaceutical for imaging phaeochromocytomas (when labelled with I123, which has a short half-life) or for treating malignant

SURGERY 32:10

Biological activity of catecholamines Dopamine in the systemic circulation acts on dopaminergic receptors in the splanchnic vessels to induce vasodilatation. The effects of noradrenaline and adrenaline are mediated by two types of receptors a and b (Table 2). A

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