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  1. 1. Alpha-Adrenergic Blockers Dr. Hiwa K. Saaed Ph.D. Pharmacology & Toxicology
  2. 2. Receptor Tissues Response α1 Eye-radial (dilatory) muscle Contacts (mydriasis) Most vascular smooth muscle -Arterioles (skin, viscera) -veins Contraction ↑TPR→↑ diastolic pressure ↑afterload ↑ venous return→↑ preload Bladder trigone and sphincter Contraction→urinary retention Male sex organs Vas deferens→ ejaculation Liver (in some species, rat ) Stimulate glycogenolysis Kidney ↓ rennin release Pilomotor smooth muscle Contraction (erect hair)
  3. 3. Receptor tissue Response α 2 Adrenergic & cholinergic Prejunctional nerve terminals ↓ transmitter release and NE synthesis Platelets Stimulate Aggregation Pancreas (β cells) Inhibit insulin release Some vascular smooth muscle Contracts Fat cells Inhibits lipolysis Medullary vasomotor center ↓sympathetic outflow → CO & vasodilation→↓ BP
  4. 4. Localization of 1-Adrenergic Receptors
  5. 5.  Like the agonists, the adrenergic antagonists are classified according to their relative affinities for α1 or α2 receptors .  Drugs that block α-adrenoceptors profoundly affect blood pressure, resulting in decreased peripheral vascular resistance.  Because normal sympathetic control of the vasculature occurs in large part through agonist actions on α-adrenergic receptors  This induces a reflex tachycardia resulting from the lowered blood pressure. Alpha-Adrenergic Blockers
  6. 6. Alpha-Adrenergic Blockers Classification I. Type of blockade 1. Irreversible (non-competitive) : Phenoxybenzamine; slow onset and long duration. 2. Reversible (competitive): All the rest II. Selectivity: α1 or α2 receptors . -Nonselective: Phenoxybenzamine and phentolamine -Selective: 1. alpha-1 selective: Prazosin, terazosin, others 2. alpha-2 selective: Yohimbine 3. alpha/beta blockers: Labetalol -Others: phenothiazines, haloperidol, tricyclic antidepressants trazodone
  7. 7. α Antagonists Receptor Affinity Prazosin, terazosin, doxazosin α1>>>>α2 Phenoxybenzamine α1>α2 Phentolamine α1=α2 Rauwolscine, yohimbine, tolazoline α2>α1 Mixed antagonists Labetalol, carvedilol β1=β2≥α1>α2
  8. 8. Alpha-Adrenergic Blockers Nonselective – Phenoxybenzamine (3-4 days) Selective • Short-acting selective 1-blocker – Prazosin t1/2 3 hrs, Alfuzosin t1/2 5 hrs • Long-acting selective 1-blockers – Terazosin t1/2 9-12 hrs – Doxazosin t1/2 22 hrs • Long-acting selective 1A-subtype – Tamsulosin t1/2 9-15 hrs
  9. 9. Pharmacological Effects - Phenoxybenzamine 1. Cardiovascular system • ↓PR: ↓ Blood pressure: reflex tachycardia: • Furthermore, the ability to block presynaptic inhibitory α2-receptors in the heart will result in more norepinephrine release, which stimulates β- receptors on the heart to increase cardiac output. • It reduces BP when sympathetic tone is high upright posture, reduced blood volume
  10. 10. Adrenergic System - Antagonists
  11. 11. Epinephrine reversal: • All α-adrenergic blockers reverse the α-agonist actions of epinephrine. the vasoconstrictive action of epinephrine is interrupted, but vasodilation of other vascular beds caused by stimulation of β2 receptors is not blocked. • Therefore, the systemic blood pressure decreases in response to epinephrine given in the presence of phenoxybenzamine. ….Why? • The actions of norepinephrine are not reversed but are diminished, …why?
  12. 12. Pharmacological Effects – Phenoxybenzamine 2. Eye – miosis 3. Nasal stuffiness 4. GI tract – Increased motility 5. Urinary bladder – decreased tone in sphincter 6. Metabolic effects – increased insulin secretion 7. It also blocks histamine, serotonin and cholinergic receptors
  13. 13. Clinical uses - Phenoxybenzamine 1. Pheochromocytoma: a catecholamine- secreting tumor of cells derived from the adrenal medulla 2. Raynaud; peripheral vascular disease 3. Autonomic hyperreflexia which predisposes paraplegics to strokes 4. BPH
  14. 14. Adverse effects Phenoxybenzamine • Postural hypotension • Tachycardia it is contraindicated in patients with decreased coronary perfusion. • Sedation, fatigue • Nasal stuffiness • Miosis • Impotence (inhibits ejaculation) • Exercise care in hypovolemic patients.
  15. 15. Imidazoline derivatives – phentolamine (5-7hr) produces a competitive block of α1 and α2- receptors. 1. Block serotonin receptors 2. Stimulate H1 & H2 3. Stimulate M1; Parasympathomimetic • Increased gastric motility & acid secretion • Increased secretion from exocrine glands, such as salivary, sweat, lacrimal, pancreatic • Cardiac stimulation reflex and direct!! • it can be injected intracavernosally to produce vasodilation of penile arteries (rarely used)
  16. 16. Alpha-1 selective blockers Prazosin, terazosin, doxazosin, alfuzosin, and tamsulosin • the first three drugs are useful in the treatment of hypertension. • Alfuzosin & Tamsulosin are indicated for the treatment of benign prostatic hypertrophy • Prazosin (t1/2 3hrs): No significant tachycardia Used in CHF and in hypertension but tolerance develops with time, may be due to fluid retention. • Adverse effects: First dose phenomenon. • Favorable effect on plasma lipids: increase HDL/LDL ratio • Terazosin (t1/2 9-12 hrs)
  17. 17. Other α -adrenoceptor antagonists • Indoramin: antihypertensive. • Urapidil is an α1 antagonist (its primary effect) that also has weak α2-agonist and 5-HT1A-agonist actions and weak antagonist action at β1 receptors. It is used in Europe as an antihypertensive agent and for BPH. • Labetalol has both α1-selective and β-antagonistic effects. • Neuroleptic drugs such as chlorpromazine and haloperidol are potent dopamine receptor antagonists but are also antagonists at α-receptors. • Antidepressant: trazodone block α1 receptors.
  18. 18. 2 - selective antagonists Yohimbine Chief active compound in Pausinystalia yohimbine (bark), Effects opposite of Clonidine Enters CNS => increased sympathetic output => increased HR, BP, can cause severe tremors Ingredient in many weight loss products Enhances sexual activity – aphrodisiac Blocks other receptors: serotonin, dopamine, Increases ADH release Experimental uses in Treatment of : 1. Raynaud's phenomenon to inhibit smooth muscle contraction, 2. type 2 diabetes (α2 receptors inhibit insulin secretion), 3. depression.
  19. 19. Therapeutic Uses of Alpha-Adrenergic Blockers • Hypertension - alpha-1 selective • Pheochromocytoma • Peripheral vascular disease – Raynaud’s • Local Vasoconstrictor Excess • Benign prostatic hyperplasia • Erectile Dysfunction: Yohimbine or intracavernous phentolamine + papaverine