a)-catecholamines // these are compounds which have the catechol nucleus as adrenaline, noradrenaline, isoprenaline, dopamine, dobutamine

Transcription

1 1 Adrenergic or sympathetic agonist: Are classified according to: I)- according to the chemical structure a)-catecholamines // these are compounds which have the catechol nucleus as adrenaline, noradrenaline, isoprenaline, dopamine, dobutamine b)-noncatecholamines // lack the catechol nucleus as ephedrine, phenylephrine, amphetamine, tyramine. II) according to their mode of action and by the spectrum of receptors that they activate : a)- directly acting agonists // these drugs act directly on α or β receptors, producing effects similar to those that occur following stimulation of sympathetic nerves or release of the hormone adrenaline from adrenal medulla for example : adrenaline, noradrenaline, dopamine, phenylephrine b)-indirect acting agonists // these agents dependents on the release of endogenous catecholamines.these indirect agents may have either of two different mechanisms : 1)-displacement of stored catecholamines from the adrenergic nerve ending (e.g tryamine, amphetamine ) 2)-inhibition reuptake of cateholamines already released (e.g the mechanism of action of cocaine and tricyclic antidepressents). c)-mixed action agonists // some against, such as ephedrine, metaraminol have capacity both to directly stimulate adrenoceptors and to release noradrenaline from the adrenergic neuron. Catecholamines : Organ-system Effects of Activation of the Adrenergic System 1. CVS: a. Heart: increased rate and force of contraction, increased cardiac output, myocardial demand, and AV conduction b. Blood Vessels and Blood pressure: constriction of blood vessels in the skin and mucous membranes. - Dilatation of skeletal muscle vessels - Adrenaline increases systolic and decreases diastolic blood pressure at low doses

2 2 but increases both at higher doses - Noradrenaline increases both systolic and diastolic blood pressure 2. Smooth Muscle: a. Bronchi: relaxation. b. Uterus: relaxation of the pregnant uterus c. GIT: relaxation of wall muscles and contraction of sphincters d. Bladder: relaxation of detrusor muscle; contraction of sphincter and trigone muscle 3. Eye: mydriasis; reduction of intraocular pressure in normal and glacucomatous eyes 4. Respiration: Bronchodilatation; relief of congestion; mild stimulation of respiration 5. Metabolic: Increased hepatic glycogenolysis; decreased peripheral glucose intake; increased free fatty acids in the blood (lipolysis) 6. CNS: excitement, vomting, restlessness 7. Skeletal muscle: facilitation of neuromuscular transmission and vasodilatation Adrenaline : (epinephrine):this is the prototype of adrenergic drugs and is produced in the body by the cells of the Adrenal medulla and by chromaffin tissues. Pharmacokinetics Adrenaline is rapidly destroyed in the gastrointestinal tract, conjugated, and oxidized in the liver. It is therefore ineffective when given orally and should be given intramuscularly or Subcutaneous. Intravenous injection is highly dangerous and is likely to precipitate ventricular fibrillation. The drug may, however, be given by nebulizer for inhalation when its relaxing effect on the bronchi is desired or it may be applied topically to mucus membranes to produce vasoconstriction. Because of the extensive metabolism of the drug in the liver, little is excreted unchanged in the urine. Pharmacodynamics Adrenaline directly stimulates all the adrenergic receptors both and brings about effects of sympathetic nerve stimulation. Its action may be divided in to two, depending on the type of receptor stimulated. The α effects consist of vasoconstriction in skin and viscera, mydriasis, platelet aggregation and some increase in blood glucose. The ß effects consists of increased contractility and rate of heart with a decreased refractory period (ß1), vasodilatation in muscles and coronary vessels (ß2), bronchial relaxation (ß2) uterine relaxation (ß2), hyperglycemia, lactic acidemia and increased circulating free fatty acids. Indications (therapeutic uses)

3 1. Acute bronchial asthma 2. Treatment of hypersensitivity reaction and Anaphylaxis shock that is characterized by bronchospasm & hypotension 3. Local haemostatic to stop bleeding in epistaxis 4. with local anesthesia to prolong the action 5. Cardiac arrest is given by intracardiac injection in a last result. 6-gluocoma (reduced intraocular pressure) Adverse reactions 1. Anxiety, restlessness, headache, tremor 2. Anginal pain 3. Cardiac arrhythmias and palpitations 4. Sharp rise in blood pressure 5. Sever vasoconstriction resulting in gangrene of extremities Contra indications 1. Coronary diseases 2. Hyperthyroidism 3. Hypertension 4. Digitalis therapy Noradrenaline: (norepinephrine, levarterenol): is an agonist at both a1 and a2 receptors. It also activates β1 receptors with similar potency as adrenaline, but has relatively little effect on β2 receptors. consequently, noradrenaline increases peripheral resistance and both diastolic and systolic blood pressure. Therapeutic uses of norepinephrine : It has very few uses 1-to treat shock: because it increases vascular resistance and therefore, increases blood pressure; however, dopamine is better, because it does not reduce blood flow to the kidney as does norepinephrine. 2-as local vasoconstrictor mixed with local anaesthetic to prolong its action because it delays its absorption into the blood stream, however epinephrine is better. Norepinephrine is never used for asthma and not used as nasal decongestant because it is not absorbed (it cause sever vasoconstriction). 3

4 Isoprenaline : (isoproterenol) : is very potent B-receptor agonist and has little effect on a- receptor.the drug has positive chronotropic and inotropic actions; because isoprenaline activates B receptors almost exclusively, it is a potent vasodilator. these actions lead to a marked increase in cardiac output associated with a fall in diastolic and mean arterial pressure and a lesser decrease or a slight increase in systolic pressure. Therapeutic uses of isoprenaline : 1- Complete heart block : it less inactivated by COMT & MAO 2- B-blockers poisoning 3- Acute asthma : it is rarely used now, because it is not selective and has side effects. Dopamine: the immediate metabolic precursor of noradrenaline, occurs naturally in the CNS in the basal ganglia where it functions as a neurotransmitter as well as in the adrenal medulla. dopamine can activate a and B adrenocetors. lower doses stimulates β 1 cardiac receptors whereas at higher doses it can cause vasoconstriction by activates a receptors. In addition D1 and D2 dopaminergic receptors, occur in the peripheral mesenteric and renal vascular beds,where binding of dopamine produces vasodilation. D2 receptors are also found on presynaptic adrenergic neurons, where their activation interferes with noradrenaline release. Therapeutic uses of dopamine : 1- Shock : dopamine is the drug of choice in shock and is given by continuous infusion.it raises the blood pressure by stimulating the heart (β 1 action) In addition, it enhances perfusion of the kidney and splanchnic areas. 2- Treatment of congestive heart failure. Dobutamine ::(B1 against): It is a synthetic direct acting catecholamine that is β 1 recetor against. it stimulates the heart with few vascular effects. It increase cardiac output with little tachycardia and does not significantly elevate oxygen consumption of myocardium. Therapeutic uses of dobutamine : 1- Ongestive heart failure 2- Shock in patient with ischemic heart disease (e.g angina pectoris ) because it does not elevate o 2 consumption by heart. 4

5 5 Selective β 1 -agonist : a)- dobutamine : mentioned before b)-xamoterol : it is a partial agonist at B1 adrenoceptor, act as agonist or antagonist according to certain cercumstances particularily level of sympathetic autonomic activity at low level of sympathetic activity, it act as an agonist so increase heart rate and contractility. - At high level of sympathetic autonomic activity, it act as antagonist. Therapeutic uses of β2-agonist : 1-vasodilators : used in peripheral vascular diseases as Nylidrin, isoxuprine 2-Uterine relaxants : relax the pregnant uterus, used in premature labour. As ritodrine 3-bronchodilators : ( treatment of asthma) as : salbutamol, terbutaline, fenoterol. Unlike isoprenaline they have longer duration of action and selectivity to B2 receptor ( so they are more preferred than isoprenaline). a- Against : a1 agonist : mainly used as vasoconstrictants as nasal decongestant, increase Blood pressure.etc. they include : 1-phenylephrine (a1 and slight β1 effect) 2-meyhoxamine (only a1 effect) 3-metaraminol (direct a1 and some indirect sympathomimitic effects) a2 agonist : include : clonidine, a-methyldopa, xylazine, detomidine, romifidine, medetomidine, dexmedetomidine the beneficial effects of a2 agonist include ::;: 1-antihypertensive effect 2-reliable sedative 3-muscle relaxation

6 6 4-anxiolysis 5-analgesia 6-anaesthetic adjuncts β -antagonist :: selective β1 antagonist : also called cardioselective β-blockers) include : atenolol, metoprolol, : useful in treatment of hypertensive patients with impaired pulmonary function. Non-selective β-antagonist : include : propranolol ( block β1 & β2receptor) and timolol Therapeutic effect of propranolol : 1- Hypertension : lowers blood pressure (B.p) in hypertension by decrease cardiac output. 2- Glaucoma : propranolol and other β-blockers particularly timolol an effective in diminishing intraocular pressure. 3- Migraine : the mechanism of this action may depend on the blockade of catecholamine induce vasodilation in the brain. 4- Hyperthyroidism (thyroid storm ) : β-blockers may be lifesaving in protecting against serious cardiac arrhythmias which occur due to widespread sympathetic stimulation. 5- Angina pectoris : propranolol decrease the o2 requirment of heart muscle and therefore is effective in reducing the chest pain 6- Myocardial infarction. a-antagonist : Non-selective a-antagonist : as phentolamine Selective a-antagonist : either a1 or a2 antagonist :

7 7 Selective a1 antagonist : as prazosin, terazosin : they are useful in the treatment of hypertension. Selective a2 antagonist : as atipamazole which mainly used to reverse the a2 against activity Yohambine : it is a weak a2 adrenoceptor blocking agent are commonly use in case of autonomic insufficiency and it also improve male sexual function.