Regulation of secretion

Secretion of growth hormone is regulated by the action of hypothalamic GHRF and modulated by somatostatin, as described above and outlined in Figure 32.2. One of the mediators of growth hormone action, insulin-like growth factor (IGF)-1, which is released from the liver (see below), has an inhibitory effect on growth hormone secretion by stimulating somatostatin release from the hypothalamus.

Growth hormone release, like other anterior pituitary secretions, is pulsatile, and its plasma concentration may fluctuate 10- to 100-fold. These surges occur repeatedly during the day and night, and reflect the dynamics of hypothalamic control. Deep sleep is a potent stimulus to growth hormone secretion, particularly in children.

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The main effect of growth hormone (and its analogues) is to stimulate normal growth and, in doing this, it affects many tissues, acting in conjunction with other hormones secreted from the thyroid, the gonads and the adrenal cortex. It stimulates hepatic production of the IGFs—also termed somatomedins—which mediate most of its anabolic actions. Receptors for IGF-1 (the principal mediator) exist on many cell types, including liver cells and fat cells.

Growth hormone stimulates the uptake of amino acids and protein synthesis, especially in skeletal muscle. IGF-1 mediates many of these anabolic effects, acting on skeletal muscle and also on the cartilage at the epiphyses of long bones, thus influencing bone growth.

Disorders of production and clinical use

Deficiency of growth hormone results in pituitary dwarfism. In this condition, which may result from lack of GHRF or a failure of IGF generation or action, the normal proportions of the body are maintained. Growth hormone is used therapeutically in patients (often children) with growth hormone deficiency and with the short stature associated with the chromosomal disorder known as Turner’s syndrome. It may also be used to correct chronic renal insufficiency in children. Satisfactory linear growth can be achieved by giving somatropin subcutaneously, six to seven times per week, and therapy is most successful when started early. Humans are insensitive to growth hormone of other species, so human growth hormone (hGH) must be used clinically. This used to be obtained from human cadavers, but this led to the spread of Creutzfeldt–Jacob disease, a prion-mediated neurodegenerative disorder (Ch. 39). hGH is now prepared by recombinant DNA technology, which avoids this risk. Human recombinant IGF-1 is also available (mecasermin) for the treatment of growth failure in children who lack adequate amounts of this hormone. hGH is also used illicitly by athletes (see Ch. 58) to increase muscle mass. The large doses used have serious side effects, causing abnormal bone growth and cardiomegaly. It has also been tested as a means of combating the bodily changes in senescence; clinical trials have shown increases in body mass, but no functional improvement.

An excessive production of growth hormone in children results in gigantism. An excessive production in adults, which is usually the result of a benign pituitary tumour, results in acromegaly, in which there is enlargement mainly of facial structures and of the hands and feet. The dopamine agonist bromocriptine and octreotide may mitigate the condition. Another useful agent is pegvisomant, a modified version of growth hormone prepared by recombinant technology that is a highly selective antagonist of growth hormone actions.

Regulation of secretion

Prolactin secretion is under tonic inhibitory control by the hypothalamus (Fig. 32.3 and Table 32.1), the inhibitory mediator being dopamine (acting on D₂ receptors on the lactotrophs). The main stimulus for release is suckling; in rats, both the smell and the sounds of hungry pups are also effective triggers. Neural reflexes from the breast may stimulate the secretion from the hypothalamus of prolactin-releasing factor(s), possible candidates for which include TRH and oxytocin. Oestrogens increase both prolactin secretion and the proliferation of lactotrophs through release, from a subset of lactotrophs, of the neuropeptide galanin. Dopamine antagonists (used mainly as antipsychotic drugs; see Ch. 45) are potent stimulants of prolactin release, whereas agonists such as bromocriptine (see below and also Chs 38 and 45) suppress prolactin release. Bromocriptine is also used in parkinsonism (Ch. 39).

Fig. 32.3 Control of prolactin secretion.

Drugs are shown in red-bordered boxes. PRF, prolactin-releasing factor(s); PRIF, prolactin release-inhibiting factor(s); TRH, thyrotrophin-releasing hormone.

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There are at least three specific receptor subtypes that bind prolactin, and these are not only found in the mammary gland but are widely distributed throughout the body, including the brain, ovary, heart and lungs. The main function of prolactin in women is the control of milk production. At parturition, when the blood level of oestrogen falls, the prolactin concentration rises and lactation is initiated. Maintenance of lactation depends on suckling (see above), which causes a 10- to 100-fold increase within 30 min.

Together with other hormones, prolactin is responsible for the proliferation and differentiation of mammary tissue during pregnancy. It inhibits gonadotrophin release and/or the response of the ovaries to these trophic hormones. This is one of the reasons why ovulation does not usually occur during breastfeeding, and it is believed to constitute a natural contraceptive mechanism.

According to one rather appealing hypothesis, the high postpartum concentration of prolactin reflects its biological function as a ‘parental’ hormone. Certainly, broodiness and nest-building activity can be induced in birds, mice and rabbits by prolactin injections. Prolactin also exerts other, apparently unrelated, actions, including stimulating mitogenesis in lymphocytes. There is some evidence that it may play a part in regulating immune responses.

Modification of prolactin secretion

Prolactin itself is not used clinically. Bromocriptine, a dopamine receptor agonist, is used to decrease excessive prolactin secretion (hyperprolactinaemia). It is well absorbed orally, and peak concentrations occur after 2 h. Unwanted reactions include nausea and vomiting. Dizziness, constipation and postural hypotension may also occur. Cabergoline and quinagolide are similar.

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Tetracosactide and ACTH have two actions on the adrenal cortex:

The main use of tetracosactide is in the diagnosis of adrenal cortical insufficiency. The drug is given intramuscularly or intravenously, and the concentration of hydrocortisone in the plasma is measured by radioimmunoassay.

Regulation of secretion and physiological role

Antidiuretic hormone released from the posterior pituitary has a crucial role in the control of the water content of the body through its action on the cells of the distal part of the nephron and the collecting tubules in the kidney (see Ch. 28). The hypothalamic nuclei that control fluid balance lie close to the nuclei that synthesise and secrete ADH.

One of the main stimuli to ADH release is an increase in plasma osmolarity (which produces a sensation of thirst). A decrease in circulating blood volume (hypovolaemia) is another, and here the stimuli arise from stretch receptors in the cardiovascular system or from angiotensin release. Diabetes insipidus is a condition in which large volumes of dilute urine are produced because ADH secretion is reduced or absent, or because of a reduced sensitivity of the kidney to the hormone.

Antidiuretic hormone receptors

There are three classes of receptor for ADH: V₁, V₂ and V₃. V₂ receptors stimulate adenylyl cyclase, which mediates its main physiological actions in the kidney, whereas the V₁ and V₃ receptors are coupled to the phospholipase C/inositol trisphosphate system.

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Pharmacokinetic aspects

ADH, as well as various peptide analogues, is used clinically either for the treatment of diabetes insipidus or as a vasoconstrictor. The analogues have been developed to (a) increase the duration of action and (b) shift the potency between V₁ and V₂ receptors.

The main substances used are vasopressin (ADH itself; short duration of action, weak selectivity for V₂ receptors, given by subcutaneous or intramuscular injection, or by intravenous infusion), desmopressin (increased duration of action, V₂-selective and therefore fewer pressor effects, can be given by several routes including nasal spray) and terlipressin (increased duration of action, low but protracted vasopressor action and minimal antidiuretic properties). Felypressin is a short-acting vasoconstrictor that is injected with local anaesthetics such as prilocaine to prolong their action (see Ch. 42).

Vasopressin itself is rapidly eliminated, with a plasma half-life of 10 min and a short duration of action. Metabolism is by tissue peptidases, and 33% is removed by the kidney. Desmopressin is less subject to degradation by peptidases, and its plasma half-life is 75 min.

Unwanted effects

There are few unwanted effects and they are mainly cardiovascular in nature: intravenous vasopressin may cause spasm of the coronary arteries with resultant angina, but this risk can be minimised if the antidiuretic peptides are administered intranasally.

Synthesis and release

Glucocorticoids are not stored in the adrenal. They are synthesised under the influence of circulating ACTH secreted from the anterior pituitary gland (Fig. 32.4) and released in a pulsatile fashion into the blood. While they are always present, there is a well-defined circadian rhythm in the secretion in healthy humans, with the net blood concentration being highest early in the morning, gradually diminishing throughout the day and reaching a low point in the evening or night. ACTH secretion itself (also pulsatile in nature) is regulated by CRF released from the hypothalamus and vasopressin from the posterior gland. The release of both ACTH and CRF, in turn, is reflexly inhibited by the ensuing rising concentrations of glucocorticoids in the blood. This functional hypothalamic–pituitary–adrenal unit is referred to as the HPA axis.

Opioid peptides also exercise a tonic inhibitory control on the secretion of CRF, and psychological factors can affect the release of both vasopressin and CRF, as can stimuli such as excessive heat or cold, injury or infections. This is the principal mechanism whereby the HPA axis is activated in response to a threatening environment.

The precursor of glucocorticoids is cholesterol (Fig. 32.5). The initial conversion of cholesterol to pregnenolone is the rate-limiting step and is itself regulated by ACTH. Some of the reactions in the biosynthetic pathway can be inhibited by drugs. Metyrapone prevents the β-hydroxylation at C11, and thus the formation of hydrocortisone and corticosterone. Synthesis is blocked at the 11-deoxycorticosteriod stage, and as these substances have no effects on the hypothalamus and pituitary, there is a marked increase in ACTH in the blood. Metyrapone can therefore be used to test ACTH production, and may also be used to treat patients with Cushing’s syndrome. Trilostane (also of use in Cushing’s syndrome and primary hyperaldosteronism) blocks an earlier enzyme in the pathway—the 3β-dehydrogenase.

Fig. 32.5 Biosynthesis of corticosteroids, mineralocorticoids and sex hormones.

All steroid hormones are synthesised from cholesterol. Successive steps of hydroxylation and dehydrogenation are important in the biosynthetic pathway and are targets for drugs. Intermediates are shown in green boxes; interconversions occur between the pathways. Blue boxes indicate circulating hormones. Drugs are shown in red-bordered boxes adjacent to their sites of action. Glucocorticoids are produced by cells of the zona fasciculata, and their synthesis is stimulated by adrenocorticotrophic hormone (ACTH); aldosterone is produced by cells of the zona glomerulosa, and its synthesis is stimulated by angiotensin II (angio II). Metyrapone inhibits glucocorticoid synthesis, aminoglutethimide and trilostane block synthesis of all three types of adrenal steroid (see text for details). Carbenoxolone inhibits the interconversion of hydrocortisone and cortisone in the kidney. Enzymes: 17-α-OH, 17-α-hydroxylase; 3-β-dehyd, 3-β-dehydrogenase; 21-β-OH, 21-β-hydroxylase; 11-β-OH, 11-β-hydroxylase; 11-β-dehyd, 11-β-hydroxysteroid dehydrogenase.

Aminoglutethimide inhibits the initial step in the biosynthetic pathway and has the same overall effect as metyrapone. Ketoconazole, an antifungal agent (Ch. 52), used in higher doses also inhibits steroidogenesis and may be of value in the specialised treatment of Cushing’s syndrome.

Mechanism of action

The glucocorticoid effects relevant to this discussion are initiated by interaction of the drugs with specific intracellular glucocorticoid receptors belonging to the nuclear receptor superfamily (although there may be other binding proteins or sites; see Norman et al., 2004). This superfamily (see Ch. 3) also includes the receptors for mineralocorticoids, the sex steroids, thyroid hormones, vitamin D₃ and retinoic acid. The actual mechanism of transcriptional control is complex with at least four mechanisms operating within the nucleus. These are summarised diagrammatically in Figure 32.6.

Fig. 32.6 Molecular mechanism of action of glucocorticoids.

The schematic figure shows three possible ways by which the liganded glucocorticoid receptor can control gene expression following translocation to the nucleus. [A] Basic transactivation mechanism. Here, the transcriptional machinery (TM) is presumed to be operating at a low level. The liganded glucocorticoid receptor (GR) dimer binds to one or more ‘positive’ glucocorticoid response elements (GREs) within the promoter sequence (shaded zone) and upregulates transcription. [B] Basic transrepression mechanism. The transcriptional machinery is constitutively driven by transcription factors (TF). In binding to the ‘negative’ GRE (nGRE), the receptor complex displaces these factors and expression falls. [C] Fos/Jun mechanism. Transcription is driven at a high level by Fos/Jun transcription factors binding to their AP-1 regulatory site. This effect is reduced in the presence of the GR. [D] Nuclear factor (NF)κβ mechanism. The transcription factors P65 and P50 bind to the NFκβ site, promoting gene expression. This is prevented by the presence of the GR, which binds the transcription factors, preventing their action (this may occur in the cytoplasm also). (For further details of the structure of the glucocorticoid receptor, see Ch. 3.)

(Modified from Oakley R H, Cidlowski J A in Goulding N J, Flower R J (eds) 2001 Glucocorticoids. Birkhauser Verlag.)

In addition to controlling gene expression, the liganded receptor itself, in either a monomeric or a dimeric form, may trigger important signal transduction events while still in the cytosolic compartment (there may even be a subpopulation of receptors that reside there permanently). One of these cytosolic effects, germane to the anti-inflammatory action of these drugs, is the release, following phosphorylation, of the protein annexin-1, which has potent inhibitory effects on leukocyte trafficking and other biological actions. The significance of such ‘receptor-mediated, non-genomic’ actions is that they can happen very rapidly (within seconds), as they do not entail changes in protein synthesis that require a longer time frame.

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General metabolic and systemic effects

The main metabolic effects are on carbohydrate and protein metabolism. The glucocorticoids cause both a decrease in the uptake and utilisation of glucose and an increase in gluconeogenesis, resulting in a tendency to hyperglycaemia (see Ch. 30). There is a concomitant increase in glycogen storage, which may be a result of insulin secretion in response to the increase in blood sugar. Overall, there is decreased protein synthesis and increased protein breakdown, particularly in muscle, and this can lead to wasting. Glucocorticoids also have a ‘permissive’ effect on the cAMP-dependent lipolytic response to catecholamines and other hormones. Such hormones cause lipase activation through a cAMP-dependent kinase, the synthesis of which requires the presence of glucocorticoids (see below). Large doses of glucocorticoids given over a long period result in the redistribution of body fat characteristic of Cushing’s syndrome (Fig. 32.7).

Fig. 32.7 Cushing’s syndrome.

This is caused by excessive exposure to glucocorticoids, and may be caused by disease (e.g. an adrenocorticotrophic hormone-secreting tumour) or by prolonged administration of glucocorticoid drugs (iatrogenic Cushing’s). Italicised effects are particularly common. Less frequent effects, related to dose and duration of therapy, are shown in parentheses.

(Adapted from Baxter JD, Rousseau G G (eds) 1979 Glucocorticoid hormone action. Monographs on Endocrinology, vol 12. Springer-Verlag, Berlin.)

Glucocorticoids tend to produce a negative calcium balance by decreasing Ca²⁺ absorption in the gastrointestinal tract and increasing its excretion by the kidney. This may contribute to osteoporosis (see below). In higher, non-physiological concentrations, the glucocorticoids have some mineralocorticoid actions (see below), causing Na⁺ retention and K⁺ loss—possibly by swamping the protective 11β-hydroxysteriod dehydrogenase and acting at mineralocorticoid receptors.

Unwanted effects

Unwanted effects occur with large doses or prolonged administration of glucocorticoids rather than replacement therapy, and are a serious problem. The major effects are as follows:

Sudden withdrawal of the drugs after prolonged therapy may result in acute adrenal insufficiency through suppression of the patient’s capacity to synthesise corticosteroids.⁵ Careful procedures for phased withdrawal should be followed. Recovery of full adrenal function usually takes about 2 months, although it can take 18 months or more.

Pharmacokinetic aspects

There are many glucocorticoid drugs in therapeutic use. Although cortisol, the endogenous hormone, is often used, synthetic derivatives are even more common. These have different physicocochemical properties as well as varying potency and have been optimised for administration by different routes. They may be administered orally, systemically or intra-articularly; given by aerosol into the respiratory tract, administered as drops into the eye or the nose, or applied in creams or ointments to the skin. Topical administration diminishes the likelihood of systemic toxic effects unless large quantities are used. When prolonged use of systemic glucocorticoids is necessary, therapy on alternate days may decrease suppression of the HPA axis and other unwanted effects.

As small lipophilic molecules, glucocorticoids probably enter their target cells by simple diffusion. Hydrocortisone has a plasma half-life of 90 min, although its main biological effects have a 2–8 h latency. Biological inactivation, which occurs in liver cells and elsewhere, is initiated by reduction of the C4–C5 double bond. Cortisone and prednisone are inactive until converted in vivo to hydrocortisone and prednisolone, respectively.

Endogenous glucocorticoids are transported in the plasma bound to corticosteroid-binding globulin (CBG) and to albumin. CBG accounts for about 77% of bound hydrocortisone, but many synthetic glucocorticoids are not bound at all. Albumin has a lower affinity for hydrocortisone but binds both natural and synthetic steroids. Both CBG-bound and albumin-bound steroids are biologically inactive.

The clinical use of systemic glucocorticoids is given in the clinical box. Dexamethasone has a special use: it can be used to test HPA axis function in the dexamethasone suppression test. A relatively low dose, usually given at night, should suppress the hypothalamus and pituitary, and result in reduced ACTH secretion and hydrocortisone output, as measured in the plasma about 9 hours later. Failure of suppression implies hypersecretion of ACTH or of glucocorticoids (Cushing’s syndrome).

Regulation of aldosterone synthesis and release

The regulation of the synthesis and release of aldosterone is complex. Control depends mainly on the electrolyte composition of the plasma and on the angiotensin II system (Fig. 32.4; Chs 22 and 28). Low plasma Na⁺ or high plasma K⁺ concentrations affect the zona glomerulosa cells of the adrenal directly, stimulating aldosterone release. Depletion of body Na⁺ also activates the renin–angiotensin system (see Fig. 22.4). One of the effects of angiotensin II is to increase the synthesis and release of aldosterone (see Fig. 28.5).

Mechanism of action

Like other steroid hormones, aldosterone acts through specific intracellular receptors of the nuclear receptor family. Unlike the glucocorticoid receptor, which occurs in most tissues, the mineralocorticoid receptor is restricted to a few tissues, such as the kidney and the transporting epithelia of the colon and bladder. Cells containing mineralocorticoid receptors also contain the 11β-hydroxysteroid dehydrogenase type 2 enzyme (see above), which converts hydrocortisone (cortisol) into inactive cortisone. This has a low affinity for the mineralocorticoid receptors, thus ensuring that the cells are appropriately affected only by the mineralocorticoid hormone itself. Interestingly, this enzyme is inhibited by carbenoxolone (previously used to treat gastric ulcers; see Ch. 29), a compound derived from liquorice. If this inhibition is marked, it allows corticosterone to act on the mineralocorticoid receptor, producing a syndrome similar to Conn’s syndrome (primary hyperaldosteronism) except that the circulating aldosterone concentration is not raised.

As with the glucocorticoids, the interaction of aldosterone with its receptor initiates transcription and translation of specific proteins, resulting in an increase in the number of sodium channels in the apical membrane of the cell, and subsequently an increase in the number of Na⁺-K⁺-ATPase molecules in the basolateral membrane (see Fig. 28.5), causing increased K⁺ excretion (see Ch. 28). In addition to the genomic effects, there is evidence for a rapid non-genomic effect of aldosterone on Na⁺ influx, through an action on the Na⁺-H⁺ exchanger in the apical membrane.

Clinical use of mineralocorticoids and antagonists

The main clinical use of mineralocorticoids is in replacement therapy. The most commonly used drug is fludrocortisone (Table 32.2 and Fig. 32.4), which can be taken orally. Spironolactone is a competitive antagonist of aldosterone, and it also prevents the mineralocorticoid effects of other adrenal steroids on the renal tubule (Ch. 28). Side effects include gynaecomastia and impotence, because spironolactone also has some blocking action on androgen and progesterone receptors. It is used to treat primary or secondary hyperaldosteronism and, in conjunction with other drugs, in the treatment of resistant hypertension and of heart failure (Ch. 22) and resistant oedema (Ch. 28). Eplerenone has a similar indication and mechanism of action, although fewer side effects.