LOCAL HORMONES January 2012 1112020 1 Local Hormones

LOCAL HORMONES January, 2012 11/1/2020 1

Local Hormones • v v Includes: Histamine serotonin ( 5 -HT ) Prostaglandins vasoactive peptides- platelet activating factor (PAF), bradykinin, nitric oxide neuropeptides, cytokines 11/1/2020 2

HISTAMINE-Synthesis, Storage & Release • Is a basic amine formed from amino acid histidine by histidine decarboxylase. • It is found in most tissues but is present in high concentrations in the lungs and the skin, and in particularly high concentration in the gastrointestinal tract. • At cellular level, it is stored in high concentrations in mast cells. • But non-mast cell histamine occurs in ‘histaminocytes’ in the stomach and in histaminergic neurons in the brain. 11/1/2020 3

HISTAMINE-Synthesis, Storage & Release…. • In mast cells and basophils, histamine is complexed in intracellular granules with an acidic protein and a high-molecular-weight heparin termed macroheparin • It is released from mast cells by exocytosis during inflammatory or allergic reactions 11/1/2020 4

Histamine-……. • It is metabolized by amine oxidase enzymes • Histaminase and/or by methylating enzyme imidazole N-methyltransferase • Excess production of histamine in the body can be detected by measurement of imidazole-acetic acid, its major metabolite, in the urine. 11/1/2020 5

Histamine… • Because it is released from mast cells in response to immunoglobulin E (Ig. E)mediated ( immediate ) allergic reactions, this autacoid plays an important pathophysiologic role in seasonal rhinitis ( hay fever ), urticaria, and angioneurotic oedema. • Histamine also plays an important role in control of acid secretion in the stomach. 11/1/2020 6

Histamine Receptors and Effects • Produces its action by an effect on specific histamine receptors which are of three main types: • H 1, H 2 and H 3 - receptors, distinguished by means of selective antagonist drugs. • H 1 and H 2 receptors mediate most of the well-defined peripheral actions 11/1/2020 7

Histamine Receptors and Effects… a) H 1 receptor: This receptor is important in smooth muscle effects, especially those caused by Ig. E-mediated responses. • Causes contractions of the smooth muscles of (the ileum, the bronchi, bronchioles and uterus) • Typical responses include bronchoconstriction and vasodilation, the latter by release of endothelium-derived relaxing factor (EDRF). 11/1/2020 8

Histamine Receptors and Effects… • Capillary endothelium, in addition to releasing endothelium-derived relaxing factor (EDRF), also contracts, opening gaps in the permeability barrier and resulting in the formation of local oedema. • These effects are manifested in allergic reactions and in mastocytosis, a rare neoplasm of mast cells. 11/1/2020 9

b) H 2 receptors: has three actions which are: • mediates gastric acid secretion by parietal cells in the stomach. Clinically, this is the most important action of histamine, since it is implicated in the pathogenesis of peptic ulcer. • It also has cardiac stimulant effect. • A third action is to reduce histamine release from mast cells: -a –ve feedback effect. These actions are mediated by activation of adenylyl cyclase, which increases intracellular c. AMP. 11/1/2020 10

Histamine Receptors and Effects… c) H 3 receptor: • This receptor appears to be involved mainly in presynaptic modulation of histaminergic neurotransmission in the CNS. • In periphery, it appears to be a presynaptic heteroceptor with modulatory effects on the release of other transmitters. 11/1/2020 11

CLINICAL USE • Histamine has no therapeutic applications, but drugs that block histamine’s effects are very important in clinical medicine 11/1/2020 12

HISTAMINE H 1 ANTAGONISTS • A wide variety of antihistaminic H 1 blockers are available from different chemical families. • Diphenhydramine and Chlorpheniramine may be considered prototypes. • Because they have been developed for use in chronic conditions, H 1 blockers are all active by the oral route. • Most are metabolized extensively in the liver. 11/1/2020 13

HISTAMINE H 1 ANTAGONISTS… • Half-lives of the older H 1 -blockers vary from 4 hours to 12 hours. • Several newer agents (“ second generation “ antihistamines, e, g. Terfenadine, Fexofenadine, Astemizole, Loratadine ) have half lives of 12 -24 hours and decreased CNS penetration. 11/1/2020 14

MECHANISM AND EFFECTS • H 1 blockers are competitive pharmacologic antagonists at H 1 receptor, these drugs have no effect on histamine release from storage sites. Because their structure closely resembles that of muscarinic blockers and alpha blockers, many of these agents are also pharmacologic antagonists at these receptors. A few also block 5 HT receptors. However, they have negligible effects at H 2 receptors. 11/1/2020 15

MECHANISM AND EFFECTS… • H 1 -blocking drugs have sedative and antimotion sickness effects in the CNS. In periphery, they competitively inhibit the effect of histamine ( especially if given before histamine release occurs ). • Many H 1 blockers are potent local anaesthetics. 11/1/2020 16

Clinical Use of Histamine H 1 antagonists • Have many application in allergies of immediate type ( i. e. those caused by antigens acting on Ig. E antibody-sensitized mast cells ). • These conditions include hay fever and urticaria. • The drugs have a broad spectrum of adverse effects that limit their usefulness but can sometimes be used to good effect ( e. g. the sedative effect is used in over-the-counter (OTC) sleep aids ). 11/1/2020 17

TOXICITY AND INTERACTIONS • Sedation is common, with Diphenhydramine and Promethazine. • It is less common with newer agents that do not enter the CNS readily ( e. g. Terfenadine, Astemizole ). • Antimuscarinic effects such as dry mouth and blurred vision occur with some drugs in some patients. 11/1/2020 18

TOXICITY AND INTERACTIONS… • Alpha-blocking actions may cause orthostatic hypotension. • Interaction occur between older antihistamines and other drugs with sedative effects, e. g. benzodiazepines and alcohol. 11/1/2020 19

• Drugs that inhibit hepatic metabolism may result in dangerously high levels of non sedating antihistaminic drugs that are taken concurrently. For e. g. Ketoconazole inhibits metabolism of astemizole and terfenadine. • In the presence of this anti-fungal drug, the plasma concentration of either antihistamine may increase and precipitate lethal arrhythmias. • Cardiac arrhythmias are effects of excessive concentrations of terfenadine and astemizole. 11/1/2020 20

• Terfenadine is the pro-drug of the active antihistaminic molecule Fexofenadine. • It is not subjected to the interaction described above and has become available as a prescription drug. 11/1/2020 21

HISTAMINE H 2 ANTAGONISTS • Cimetidine is the prototype. • These drugs do not resemble H 1 blockers structurally. • They are orally active, with half-lives of 1 -3 hours. • Because they are relatively non-toxic, they can be given in large doses, so that the duration of action of a single dose may be 12 -24 hours. 11/1/2020 22

HISTAMINE H 2 ANTAGONISTS MECHANISM AND EFFECTS • These drugs produce a surmountable pharmacologic blockade of histamine H 2 receptors. • They are relatively selective and have no significant blocking actions at H 1 or autonomic receptors. 11/1/2020 23

HISTAMINE H 2 ANTAGONISTS MECHANISM AND EFFECTS… • The only therapeutic effect of clinical importance is the reduction of gastric acid secretion, but this is a very useful action. • Blockade of cardiovascular H 2 receptormediated effects can be demonstrated but has no clinical significance. 11/1/2020 24

HISTAMINE H 2 ANTAGONISTS CLINICAL USE • In acid-peptic disease, especially duodenal ulcer, these drugs reduce symptoms, accelerate healing, and prevent recurrences. • Acute ulcer is usually treated with 2 or more doses per day, while recurrence of the ulcer can often be prevented with a single bed-time dose. 11/1/2020 25

HISTAMINE H 2 ANTAGONISTS TOXICITY • Cimetidine is a potent inhibitor of hepatic drug metabolizing enzymes and may reduce hepatic blood flow. • It also has significant anti-androgen effects in many patients. • Ranitidine has weaker inhibitory effect on hepatic drug metabolism; neither it nor the newer H 2 blockers appear to have endocrine effects. 11/1/2020 26

SEROTONIN ( 5 -Hydroxytryptamine, 5 -HT ) Revision: • Discuss the distribution, biosynthesis, storage, release, re-uptake and degradation of 5 -HT • What are the actions and functions of 5 -HT? • Classify the main 5 -HT receptor subtypes; in each state the location, main effects, agonists and antagonists 11/1/2020 27

5 -HT-Revision… • What are the important drugs that act on 5 -HT receptors in the periphery? 11/1/2020 28

SEROTONIN ( 5 -Hydroxytryptamine, 5 -HT ) and Related Agonists • Serotonin is produced from tryptophan and stored in the enterochromaffin cells of the gut and in the CNS. • Degradation of 5 -HT occurs mainly through oxidative deamination, catalysed by monoamine oxidase (MAO), followed by oxidation to 5 -hydroxy-indoleacetic acid (5 -HIAA), which is excreted in urine. 11/1/2020 29

• Excess production of serotonin in the body can be detected by measuring its major metabolite, 5 -hydroxyindoleacetic acid ( 5 -HIAA ), in the urine. 11/1/2020 30

Serotonin (5 -HT)… • 5 -HT appears to play a physiologic enteric role as a neurotransmitter in both the CNS and the nervous system and perhaps as a local hormone that modulates gastrointestinal tract activity. • 5 -HT is also stored ( but synthesized to only a minimal extent ) in platelets. • Only one drug is in use for its 5 -HT agonist effects i. e. specific agonist. (Revision-What is it? Its use? ? ) 11/1/2020 31

Serotonin (5 -HT)… • Answer: The drug is Sumatriptan, a 5 HT 1 D receptor agonist used to treat migraine. • Others include: Buspirone used in anxiety; Ø 5 -HT 4 receptor agonist metoclopramide used to stimulate gastric emptying. Ø Ergotamine is a partial agonist 11/1/2020 32

Serotonin (5 -HT)… • Several drugs are in use or under investigation as 5 -HT antagonists. (Revision- E. g? ? ? Use? ? ? 11/1/2020 33

5 -HT Receptor Antagonists Revision • 5 -HT 3 -receptor antagonists (e. g. ondansetron, granisetron; tropisetron) used as anti-emetic drugs particularly for controlling the severe nausea and vomiting that occurs with many forms of cancer chemotherapy. 11/1/2020 34

5 -HT Receptor Antagonists Revision… • 5 -HT 2 -receptor antagonists (e. g. dihydroergotamine, methysergide, cyproheptadine, kentanserin, ketotifen, pizotifen). These ‘classical’ 5 -HT antagonists act mainly on the 5 -HT 2 receptors. They are, however nonselective, and act also on targets, such as alpha-adrenoceptors and histamine receptors. 11/1/2020 35

5 -HT Receptor Antagonists Revision… • Dihydrergotamine and methysergide belong to the ergot family and are used mainly for migraine prophylaxis. Ketotifen is sometimes used to treat asthma but the role of 5 -HT receptors in this condition is unclear. Other 5 -HT 2 antagonists are used to control the symptoms of carcinoid tumours. 11/1/2020 36

RECEPTORS AND EFFECTS • 5 -HT 1 receptors: -Are most important in the brain and mediate synaptic inhibition via increased potassium conductance. • Peripheral 5 -HT 1 receptors mediate both excitatory and inhibitory effects in various smooth muscle tissues. 11/1/2020 37

• 5 HT 2 receptors: Are important in both brain and peripheral tissues. • These receptors mediate synaptic excitation in the CNS and smooth muscle contraction ( gut, bronchi, uterus, vessels ) or dilation ( vessels). • The mechanism involves ( in different tissues ) decreased potassium conductance, decreased c. AMP, and increased inositol triphosphate (IP 3). • In carcinoid tumor, this receptor probably mediates some of the flushing, diarrhoea, and bronchoconstriction characteristic of the 11/1/2020 38 disease.

• 5 HT 3 receptors: Are found in the CNS, especially in the chemoreceptive area and vomiting centre, in peripheral sensory and enteric nerves. • These receptors mediate excitation via a 5 -HTgated cation channel ( i. e, the mechanism of serotonin at the 5 -HT 3 receptors resembles that of Ach at nicotinic cholinergic cation channels ). • Antagonists acting at this receptor have proved to be of useful anti-emetic drugs. 11/1/2020 39

CLINICAL USE • Serotonin has no clinical applications. NB • Clinical conditions in which 5 -HT plays a role: there are two situations in which the peripheral actions of 5 -HT are believed to be important, are migraine and carcinoid syndrome 11/1/2020 40

OTHER SEROTONIN AGONISTS • Sumatriptan, a receptor agonist: It is a 5 HT 1 D agonist. • Effective in the treatment of acute migraine and cluster headache attacks- an observation that supports the association of 5 -HT abnormalities with these headache syndromes. • It is available for oral, nasal, or parenteral administration. 11/1/2020 41

SEROTONIN RE-UPTAKE INHIBITORS • Used for treatment of depression. • Dexfenfluramine, a reuptake inhibitor with other effects, was used exclusively for its appetite-reducing effect. While effective as an anorexiant, it is toxic: cardiac disease in patients and neurological damage in animals were reported. 11/1/2020 42

5 -HT-ANTAGONISTS • Ketanserin is a 5 HT 2 and alphaadrenoceptor blocker. • Phenoxybenzamine- an alphaadrenoceptor blocker and Cyproheptadine a H 1 -blocker are also good 5 -HT 2 blockers. • Ondansetron is a 5 -HT 3 blocker. 11/1/2020 43

MECHANISMS AND EFFECTS of 5 -HT-ANTAGONISTS • Ketanserin and Cyproheptadine are competitive pharmacological antagonists. • Phenoxybenzamine is an irreversible blocker. 11/1/2020 44

CLINICAL USE • Used in the treatment of carcinoid tumour, a neoplasm that secrete large amount of 5 -HT ( and peptides ) and causes diarrhoea, bronchoconstriction, flushing. • Ondansetrone is extremely useful in the control of post-operative vomiting associated with cancer chemotherapy. 11/1/2020 45

TOXICITY • Adverse effects of Ketanserin are those of alpha blockade and H 1 blockade. (H/W what are they? ) • The toxicity of Ondansetron include diarrhoea and headache. 11/1/2020 46

ERGOT ALKALOIDS • The ergot alkaloids are partial agonists at alpha adrenoceptors and 5 -HT receptors. • The balance of agonist-versus-antagonist effect varies from compound to compound. • Some are also agonists at the dopamine receptor. 11/1/2020 47

CLASSIFICATION AND PROTOTYPES • The ergot alkaloids may be subdivided into 3 major groups on the basis of the organ or tissue in which they have their primary effects. • This division is not absolute, since most of the alkaloids have some effects on several tissues. 11/1/2020 48

• The brain is a target organ for several natural ergot alkaloids that cause the hallucinations and chemical psychoses associated with epidermics of ergotism. • The most important derivatives acting in the CNS are the semisynthetic drugs lysergic acid diethylamide-LSD and Bromocriptine. • The uterus is very sensitive to ergot alkaloids as term pregnancy nears but less so at other times. 11/1/2020 49

• Ergonovine is a prototypical oxytocic ergot alkaloid. • Blood vessels are sensitive to ergonovine and other ergot drugs ergotamine is a prototype 11/1/2020 50

EFFECTS • VESSELS: Ergot alkaloids can produce marked and prolonged alpha receptormediated vasoconstriction. • An overdose can cause ischaemia and gangrene of the limbs. 11/1/2020 51

EFFECTS • UTERUS: A powerful contraction occurs in this tissue near term. This is sufficient to cause abortion or miscarriage. • After delivery of the placenta, ergonovine or ergotamine can produce a useful contraction of the uterus that reduces blood loss. 11/1/2020 52

• BRAIN: Hallucinations may be prominent with the naturally occurring ergots and with lysergic acid diethylamide (LSD) but less common with therapeutic ergots. • Although LSD is potent 5 -HT 2 blocker in peripheral tissues, its CNS effects may be due to actions at dopamine receptors. • In pituitary, some ergot alkaloids are potent dopamine-like inhibitors of prolactin secretion. • Bromocriptine and Pergolide are among the most potent of the semisynthetic derivatives at these dopamine D 2 receptors and at similar 11/1/2020 53 receptors in the basal ganglia.

CLINICAL USES • Ergotamine is a mainstay of treatment of acute attacks of migraine. • Methysergide and ergonovine are used prophylactically. • Obstetric bleeding: Ergonovine and ergotamine are effective agents for reduction of postpartum bleeding 11/1/2020 54

• Hyperprolactinemia and Parkinsonism: Bromocripine and Pergolide are used to reduce prolactin secretion ( dopamine is the physiologic prolactin release inhibitor ). • Bromocriptine also reduce the size of pituitary tumours of prolactin-secreting cells. This drug is also useful in the treatment of Parkinson’s disease. 11/1/2020 55

OTHER USES • Methysergide has been used in carcinoid tumour. 11/1/2020 56

TOXICITY • VASCULAR EFFECTS Severe prolonged vasoconstriction can result in ischemia and gangrene. • The only consistently effective antagonist is Nitroprusside 11/1/2020 57

• Methysergide produces an unusual hyperplasia of connective tissue. • Similar lesions are found in some patients with characinoid, suggesting that this action is probably mediated by agonist effects at serotonin receptors. 11/1/2020 58

GASTROINTESTINAL EFFECTS: They cause nausea, vomiting, diarrhoea in many individuals. UTERINE EFFECTS; Marked uterine contractions may be produced. • Although abortion due use of ergot for migraine is rare, most obstetricians recommend avoidance. 11/1/2020 59

CNS EFFECTS • Hallucination resembling psychosis are common with LSD but less common with other ergot alkaloids. 11/1/2020 60

VASOACTIVE PEPTIDES • Include angiotensin, bradykinin, atrial natriuretic peptide, vasoactive intestinal peptide, substance P, calcitonin generelated peptide, vasopressin, glucagons, and several opioid peptides 11/1/2020 61

MECHANISMS • These agents probably all act on cell surface receptors. • Most act via G protein-coupled receptors and cause the production of second messengers; a few may open ion channels. 11/1/2020 62

ANGIOTENSIN AND ITS ANTAGONISTS SOURCE AND DISPOSITION; • Angiotensin I is produced from angiotensinogen by renin, a proteolytic enzyme released from the juxtaglomerular apparatus of the kidney. 11/1/2020 63

• An inactive decapeptide, angiotensin I is converted into angiotensin II, an octapeptide, by angiotensin-converting enzyme (ACE), also known as peptidyl dipeptidase or kinisase II. • Angiotensin II, the active form of peptide, is rapidly degraded by peptidases (angiotensinases ). • H/W- Read on Renin-Angiotensin system 11/1/2020 64

EFFECTS • Angiotensin II is potent arteriolar vasoconstrictor and stimulant of aldosterone release. • A-II directly increases peripheral vascular resistance and, through aldosterone, causes renal sodium retention. • A-II also facilitates the release of noradrenaline from adrenergic nerve endings via pre-synaptic hetero-receptor action. 11/1/2020 65

CLINICAL ROLE • In the past it was used to control bleeding by intra-arterial in sites difficult to access. • Its major clinical significance is as a pathophysiologic mediator in some cases of hypertension ( high-renin hypertension ) and in congestive heart failure. 11/1/2020 66

ANTAGONISTS • Two types of antagonists are available. • Angiotensin II receptor-blockers ( e, g, losartan, valsartan ) are orally active nonpeptide inhibitors at the angiotensin II AT 1 receptor. 11/1/2020 67

• Saralasin, a peptide a partial agonist at this receptor, is not used clinically. • Angiotensin-converting enzyme inhibitors (e. g, captopril, enalapril ) are important agents for the treatment of hypertension and heart failure. 11/1/2020 68

• Blocking the effects of angiotensin by either of these drug types is often accompanied by a compensatory increase in renin and angiotensin I. 11/1/2020 69

BRADYKININ SOURCE AND DISPOSITION: • Is one of several vasodilator kinins produced from kininogen by a family of enzymes, the kallikreins. • Bradykinin is rapidly degraded by various peptidases, including angiotensinconverting enzymes. 11/1/2020 70

EFFECTS • Bradykinin is one of the most potent vasodilator known. The peptide is thought to be inyolved in inflammation and cause oedema and pain when released or injected into tissue. • Bradykinin can be found in saliva and may play a role in stimulating its secretion. 11/1/2020 71

CLINICAL ROLE • Although it has no therapeutic application, bradykinin may play a role in the antihypertensive action of angiotensinconverting inhibitors. • There are no clinically important bradykinin antagonists. 11/1/2020 72

ATRIAL NATRIURETIC PEPTIDE: [ ANP ] SOURCE AND DISPOSITION • ANP also known as atrial natriuretic factor is synthesized and stored in the cardiac atria of mammals. • ANP is released from the atria in response to distention of the chambers. ( Revision-release of ANP occurs during volume overload in response to stretching of the atria) 11/1/2020 73

EFFECTS of ANP • ANP activates guanlyl cyclase in many tissues. • It is a vasodilator as well as natriuretic (sodium excretion-enhancing ) agent. • Its renal action includes increased glomerular filtration, decreased proximal tubular sodium reabsorption, and inhibitory effects on renin secretion. 11/1/2020 74

EFFECTS of ANP • The peptide also inhibits the action of angiotensin II and aldosterone. • Although it lacks positive inotropic action, atrial natriuretic peptide may play an important compensatory role in congestive cardiac failure by limiting sodium retention. 11/1/2020 75

CLINICAL ROLE • ANP has been studied for possible use in the treatment of congestive cardiac failure. • There are no clinically important products that act as agonist or antagonist at ANP receptors. 11/1/2020 76

ENDOTHELIN • Are peptide vasoconstrictors formed and released by endothilin cells in blood vessels. • Endothelins are believed to function as paracrine hormones in vasculature. • Three different endothelin peptides [ ET-1, ET-2, and ET-3 ] with minor variations in amino acid sequence have been identified, both of which are G protein-coupled. 11/1/2020 77

• Endothelin are much more potent than NA as vasoconstrictors and have a relatively longlasting effect. • The peptide stimulates the heart, increase atrial natriuretic peptide release, and activate smooth muscle proliferation. • The peptides may be involved in some form of hypertensions and other cardiovascular disorders. • Antagonists have recently become available for research use. 11/1/2020 78

VASOACTIVE INTESTINAL PEPTIDE, SUBSTANCE P, CALCITONIN GENE-RELATED PEPTIDE AND NEUROPEPTIDE Y Vasoactive intestinal peptide • VIP is an extremely potent vasodilator but probably is physiologically more important as a transmitter. • It is found in the central and peripheral nervous systems and in the G. I. T. • No clinical application has been found for this peptide. 11/1/2020 79

Substance P • Substance P is another neuro-transmitter peptide with potent vasodilator action on arterioles. • However, substance P is a potent stimulant of veins and of intestinal and airway smooth muscle. 11/1/2020 80

Substance P… • The peptide may also function as a local hormone in the G. I. T. • Highest concentrations of substance P are found in those parts of the nervous system that contain neurons subserving pain. 11/1/2020 81

Substance P… • At the present time, there are no clinical applications for substance P or its antagonists. • However, capsaicin, the ‘hot’ component of chilli peppers, release substance P from its stores in nerve endings and depletes the peptide. • Capsaicin has been approved for topical use on arthritic joints. 11/1/2020 82

Calcitonin Gene Related Peptide • Calcitonin gene-related peptide ( CGRP ) is found in high concentrations in the thyroid and is also present in most smooth muscle tissues. • The presence of CGRP in smooth muscle suggests a function as a co-transmitter in autonomic nerve endings. 11/1/2020 83

Calcitonin Gene Related Peptide • CGRP is the most potent hypotensive agent discovered to date and cause reflex tachycardia. • There is no clinical application for this peptide at present. 11/1/2020 84

Neuropeptide Y • Unlike three peptides described above, (i. e. VIP, Substance P, CGRP), neuropeptide Y ( NPY ) is a potent vasoconstrictor that stimulate the heart. • NPY is found in both the CNS and the peripheral nerves. • In the periphery, NPY is most commonly localized as a co transmitter in adrenergic nerve endings. 11/1/2020 85

Further reading on: • Prostaglandins • Platelet activating factor • Nitric oxide (revision) • neuropeptides • Cytokines 11/1/2020 86