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Adrenoceptor Blockers

Adrenoceptor Blockers. Dumrongsak Pekthong M.Sc.(Pharmacology). Wording. Adrenoceptor Blocker Adrenergic Antagonist Subgroups in Sympathoplegic drugs Alpha Blocker, Alpha Antagonist Beta Blocker, Beta Antagonist. Objectives.

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Adrenoceptor Blockers

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  1. Adrenoceptor Blockers Dumrongsak Pekthong M.Sc.(Pharmacology)

  2. Wording • Adrenoceptor Blocker • Adrenergic Antagonist • Subgroups in Sympathoplegic drugs • Alpha Blocker, Alpha Antagonist • Beta Blocker, Beta Antagonist

  3. Objectives 1. Describe the effects of E and NE in the presence and in the absence of Alpha Blocker. 2. Compare the effects among Beta Blockers 3. Compare the pharmacokinetics among Beta Blockers 4. Describe the clinical applications and toxicity of typical Alpha- and Beta Blockers.

  4. Outline I. Concepts II. Alpha-Blocking Drugs A. Classification B. Pharmacokinetics C. Mechanism of Action D. Effects

  5. Outline II. Alpha-Blocking Drugs (cont’d) E. Clinical Uses F. Adverse Effects III. Beta-Blocking Drugs A. Classification and Mechanisms B. Effects and Clinical Uses C. Adverse Effects

  6. I. Concepts • Classification is based on receptor selectivity. • These drugs differ markedly in their effects and clinical applications.

  7. II. Alpha-Blocking Drugs A. Classification • based on: selective affinity for alpha receptors, reversibility 1. Irreversible, long-acting alpha blockers 2. Reversible, short-acting alpha blockers 3. Alpha1-selective blockers 4. Alpha2-selective blockers

  8. A. Classification 1. Irreversible alpha blockers : Phenoxybenzamine • slightly a 1 -selective, long-acting 2. Reversible alpha blockers: Phentolamine (nonselective), tolazoline (slightly a 2-selective) 3. a 1 blockers: Prazosin, Doxazosin, Terazosin 4. a 2 blockers: Yohimbine, rauwolscine • used primarily in researches

  9. B. Pharmacokinetics • All active orally as well as parenterally • Phenoxybenzamine: short t1/2 but long duration-48 hr (covalent bond) • Phentolamine, tolazoline: parenteral, duration 20-40 min by parenteral route • Prazosin: oral, duration 8-10 hr

  10. C. Mechanism of Action • Phenoxybenzamine: binds covalently--irreversible (insurmountable) blockade (slightly a 1 -selective) • Other agents: competitive antagonists--the effects can be overcome by increased concn of agonist

  11. D. Effects of Alpha Blockers 1. Nonselective alpha blockers • block alpha-mediated sympathetic responses and exogenous sympathomimetics • Most important effects: CVS effects • vasodilation --reduce arterial and venous pressure (a 1) • no significant direct cardiac effects

  12. D. Effects of Alpha Blockers 1. Nonselective alpha blockers (cont) • Cause reflex tachycardia (due to decreased MAP) • Tachycardia may be exaggerated because a 2 receptors are also blocked. • e.g. phenoxybenzamine, phentolamine, tolazoline

  13. D. Effects of Alpha Blockers 2. Selective a 1 blockers • The same effects as nonselective alpha blockers • But cause much less tachycardia than nonselective blocker • e.g. Prazosin, Doxazosin, Terazosin

  14. Epinephrine Reversal • occur when alpha blockers are given before Epi ---> Epi produce the opposite effects : decreased BP resulting from b 2 effect (a 1 ,a 2,b 1 ,b 2)

  15. Antagonistic effect of alpha blocker on pretreatment with alpha agonist

  16. E. Clinical Uses 1. Nonselective alpha-blockers • Presurgery of pheochromocytoma: phenoxybenzamine • During surgery: phentolamine (sometimes) • Carcinoid tumor: phenoxybenzamine (5-HT blocking) • Mastocytosis: phenoxybenzamine (H1 antihistamine) • Accidental local infiltration of alpha agonist: phentolamine • Overdose of sympathomimetics (amphetamine, cocaine, phenylpropranolamine) • Raynaud’ s phenomenon, erectile dysfunction (phentolamine)

  17. E. Clinical Uses 2. Selective a 1-blockers • Prazosin and others • Essential Hypertension • Urinary hesitancy • Prevention of urinary retention in benign prostatic hyperplasia (BPH)

  18. F. Adverse effects of Alpha blockers • Orthostatic hypotension (venodilatation) • Reflex tachycardia (nonselective > selective) • First dose hypotension (take before going to bed) • Nausea/vomiting • Caution in patients with coronary artery disease (CAD or CHD): angina

  19. III. Beta-Blocking Drugs A. Classification and Mechanisms • All are competitive antagonists • Propranolol is prototype • Classification is based on • Beta subtypes selectivity • Partial agonist activity • Lipid solubility • Local anesthetic action

  20. A. Classification and Mechanisms 1. Receptor selectivity • b 1-selective: metoprolol, atenolol • b 2 -selective: butoxamine (research only) • Nonselective: propranolol • Combined beta- and alpha-blocking: labetalol

  21. A. Classification and Mechanisms 2. Partial agonist activity • Intrinsic sympathomimetic activity, ISA • eg, pindolol, acebutolol • may be useful in patients with asthma

  22. A. Classification and Mechanisms 3. Local anesthetic activity (membrane-stabilizing activity): • disadvantage when used topically in the eye • timolol: no this activity 4. Lipid solubility • responsible for CNS adverse effects: propranolol

  23. Pharmacokinetics of Beta blockers • For systemic effects, developed for chronic oral use • Esmolol: short-acting--only used parenterally • Nadolol: longest-acting • Atenolol, acebutolol are less lipid-soluble

  24. B. Effects and Clinical Uses • Predict from beta blockade • decreased HR, force of contraction • decreased A-V conduction • slow firing rate of SA node • Cardiovascular and ophthalmic applications are extremly important

  25. B. Clinical Uses • CVS: hypertension, angina pectoris, arrhythmia prophylaxis after MI, supraventricular tachycardias, hypertrophic cardiomyopathy, congestive heart failure* • Glaucoma: reduce aqueous humor secretion (timolol)

  26. B. Clinical Uses • Migraine: propranolol • Thyroid storm, thyrotoxicosis: propranolol • Famillial tremor, other types of tremor, “stage fright” : propranolol

  27. C. Adverse effects • CVS: bradycardia, A-V blockade, congestive heart failure • Patients with airway disease: asthmatic attack • Mask sign of hypoglycemia in diabetic patients: tachycardia, tremor, anxiety • CNS effects: sedation, fatigue, sleep alterations

  28. Drug List Alpha-blockers • Nonselective: phenoxybenzamine*, phentolamine* • a 1 -selective: prazosin*, terazosin, doxazosin • a 2 -selective: yohimbine

  29. Drug List Beta-blockers • Nonselective: propranolol*, timolol, nadolol • Combined a - and b - blocking: carvedilol, labetalol • b 1-selective: metoprolol, atenolol • b 2 -selective: butoxamine

  30. The End

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