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Transcript of Adrenergic Antagonists
Adrenergic AntagonistsThese drugs act by either reversibly or irreversibly attaching to the receptor, thus preventing its activation by endogenous catecholamines.
Adrenergic antagonists are classified according to their relative affinities for or receptors in the peripheral nervous system.
-ADRENERGIC BLOCKING AGENTS adrenoceptors blockers profoundly affect blood pressure. Decrease in TPRThis induces a reflex tachycardia. Effects are more profound in an individual who is standing and less in a person who is supine. A. PhenoxybenzamineNonselective: 1- and 2-receptors.
Blocks acetylcholine, histamine, and serotonin receptors
The block is irreversible and noncompetitive.
Long-acting: last about 24 hours after a single administration.
1. Actions: Cardiovascular effects:
Alpha 1 action: It prevents vasoconstriction of peripheral blood vesselsThe decreased peripheral resistance provokes a reflex tachycardia.
Presynaptic 2 receptors in the heart: increases cardiac output. Unsuccessful in maintaining lowered blood pressure in hypertension, and its use has been discontinued for this purpose.
2. Therapeutic uses Phenoxybenzamine:Treatment of pheochromocytoma.
Phenoxybenzamine is sometimes effective in treating Raynaud disease, frostbite, and acrocyanosis.
Management of autonomic hyperreflexia.
3. Adverse effects of Phenoxybenzamine: Postural hypotensionnasal stuffinessNauseaVomitingIt may inhibit ejaculation. It may induce reflex tachycardia.
B. PhentolamineCompetitive blocker of 1 and 2 receptors.
Duration of action: approximately 4 hours
Potent vasodilator, but induces pronounced reflex tachycardia
blocking the 2 receptors of the cardiac sympathetic nerves causes cardiac stimulation
Phentolamine is used for: the short-term management of pheochromocytoma. used locally to prevent dermal necrosis and extravasation due to norepinephrine administration used to treat hypertensive crisis due to: abrupt withdrawal of clonidine from ingesting tyramine-containing foods in patients taking nonselective monoamine oxidase inhibitors.
C. Selective competitive blockers of the 1 receptor PrazosinTerazosinDoxazosinTamsulosinAlfuzosin
Prazosin, terazosin, doxazosin: treatment of hypertension. Tamsulosin and alfuzosin: treatment of benign prostatic hypertrophy (BPH).
Doxazosin is the longest acting of these drugs.1. Cardiovascular effects: They decrease peripheral vascular resistance and lower arterial blood pressure by causing the relaxation of both arterial and venous smooth muscle.
They cause minimal changes in: cardiac outputrenal blood flowglomerular filtration rate. 2. Therapeutic uses:Treatment of hypertensionThe first dose of these drugs produces an exaggerated orthostatic hypotensive response that can result in syncope (fainting). This action, termed a first-dose effect, may be minimized by: adjusting the first dose to one-third or one-fourth of the normal dose giving the drug at bedtime. These drugs improve lipid profiles and glucose metabolism in hypertensive patients. Prazosin and others are used to treat congestive heart failure. By dilating both arteries and veins, these agents decrease preload and afterload, leading to an increase in cardiac output and a reduction in pulmonary congestion.
Tamsulosin is an inhibitor (with some selectivity) of the 1A receptors found on the smooth muscle of the prostate.
3. Adverse effects: DizzinessA lack of energynasal congestionHeadacheDrowsinessorthostatic hypotension.
Prazosin Should be given along with a diuretic : retain sodium (Na+) and fluid. Tamsulosin has a caution about floppy iris syndrome, a condition in which the iris swells out in response to intraoperative eye surgery.
D. Yohimbine Selective competitive 2 blockerIt is found as a component of the bark of the yohimbe
Sometimes used as a sexual stimulant.
Yohimbine works at the level of the CNS to increase sympathetic outflow to the periphery.
21III. -ADRENERGIC BLOCKING AGENTSCompetitive antagonists.
Non-selective: 1 and 2 receptors
Cardioselective antagonists primarily block 1 receptors blockers are effective in treating Anginacardiac arrhythmiasmyocardial infarctioncongestive heart failureHyperthyroidismGlaucomaprophylaxis of migraine headaches. The names of all blockers end in -olol except for labetalol and carvedilol.22A. Propranolol: prototype -adrenergic antagonist A nonselective antagonist: blocks both 1 and 2 receptors with equal affinitySustained-release preparations for once-a-day dosing are available.1. Actions: Cardiovascular: Diminishes cardiac output:having negative inotropic and chronotropic effects . It directly depresses sinoatrial and atrioventricular activity. b. Peripheral vasoconstriction: Blockade of receptors prevents 2-mediated vasodilation. CO BP reflex peripheral vasoconstriction.
No postural hypotension occurs, because the 1-adrenergic receptors that control vascular resistance are unaffected. Bronchoconstriction: This can precipitate a respiratory crisis in patients with chronic obstructive pulmonary disease (COPD) or asthma.
d. Increased Na+ retention: Reduced blood pressure causes a decrease in renal perfusion, resulting in an increase in Na+ retention and plasma volume. In some cases, this compensatory response tends to elevate the blood pressure. For these patients, blockers are often combined with a diuretic to prevent Na+ retention
e. Disturbances in glucose metabolism: Blockade leads to decreased: Glycogenolysisglucagon secretion. Therefore, if a patient with type 1 diabetes is to be given propranolol, very careful monitoring of blood glucose is essential, because pronounced hypoglycemia may occur after insulin injection. Blockers also attenuate the normal physiologic response to hypoglycemia.Because of its ability to suppress glycogenolysis and mask tachycardia, propranolol must be used with caution by diabetic patients."
2. Pharmacokinetics:After oral administration, propranolol is almost completely absorbed.
It is subject to first-pass effect, and only about 25 percent of an administered dose reaches the circulation.
it is highly lipophilic and readily crosses the blood-brain barrier.
Propranolol is extensively metabolized, and most metabolites are excreted in the urine.
3. Therapeutic effects:a. Hypertension: by several different mechanisms of action. Decreased cardiac output is the primary mechanisminhibition of renin release from the kidneydecrease in total peripheral resistance with long term usedecreased sympathetic outflow from the CNSb. Migraine: Propranolol is also effective in reducing migraine episodes when used prophylactically. Blockers decrease the incidence and severity of the attacks.c. Hyperthyroidism: Propranolol and other blockers are effective in blunting the widespread sympathetic stimulation that occurs in hyperthyroidism. In acute hyperthyroidism (thyroid storm), blockers may be lifesaving in protecting against serious cardiac arrhythmias.d. Angina pectoris: Propranolol decreases the oxygen requirement of heart muscle and, therefore, is effective in reducing the chest pain on exertion that is common in angina. Propranolol is, thus, useful in the chronic management of stable angina but not for acute treatment. e. Myocardial infarction: Administration of a blocker immediately following a myocardial infarction reduces infarct size and hastens recovery. Used prophylactically after an MI.Protective effect on the myocardium: blocking of the actions of circulating catecholamines, which would increase the oxygen demand in an already ischemic heart muscle. Propranolol also reduces the incidence of sudden arrhythmic death after myocardial infarction. 4. Adverse effects:
The reasons for this are not clear and may be independent of -receptor blockade.The blockers must be tapered of gradually for at least a few weeks. Metabolic disturbances: Blockade: decreases glycogenolysis and glucagon secretion. Fasting hypoglycemia may occur. In addition, blockers can prevent the counter regulatory effects of catecholamines during hypoglycemia. The perception of symptoms such as tremor, tachycardia, and nervousness are blunted.
Patients administered nonselective blockers have: increased low-density lipoproteinincreased triglyceridesreduced high-density lipoprotein e. CNS effects: Propranolol has numerous CNS-mediated effects, including: DepressionDizzinessLethargyFatigueWeakness
visual disturbancesHallucinationsshort-term memory lossemotional liabilityvivid dreams (including nightmares)decreased performanceinsomnia. f. Drug interactions: Drugs that interfere with, or inhibit, the metabolism of propranolol, such as cimetidine, fluoxetine, paroxetine, and ritonavir, may potentiate its antihypertensive effects.
Conversely, those that stimulate or induce its metabolism, such as barbiturates, phenytoin, and rifampin, can decrease its effects.
B. Timolol and nadolol: Nonselective antagonistsMore potent than propranolol.
Nadolol has a very long duration of action.
Timolol reduces the production of aqueous humor in the eye. It is used topically in the treatment of chronic open-angle glaucoma.
1. Treatment of glaucoma: Topically applied agents: timolol, betaxolol, or carteolol
They decrease the secretion of aqueous humor by the ciliary body. Chronic use : The blockersAcute attack of glaucoma: pilocarpine is the drug of choice.
Onset: 30 minutesDuration of action: 12 to 24 hours.
C. Selective 1 antagonists:AcebutololAtenololMetoprololBisoprololBetaxololNebivololEsmololT