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Alpha-Adrenergic Blockers

Alpha-Adrenergic Blockers. Type of blockade Phenoxybenzamine – non-competitive; slow onset and long duration. 2-stage blockade. All the rest: competitive

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Alpha-Adrenergic Blockers

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  1. Alpha-Adrenergic Blockers • Type of blockade Phenoxybenzamine – non-competitive; slow onset and long duration. 2-stage blockade. All the rest: competitive • Selectivity Nonselective: Phenoxybenzamine and phentolamine alpha-1 selective: Prazosin, terazosin, others alpha-2 selective: Yohimbine alpha/beta blockers: Labetalol • Others: phenothiazines, tricyclic antidepressants

  2. Phenoxybenzamine Prazosin Yohimbine

  3. EPI Receptors no longer available EPI + Phenoxybenzamine Phenoxybenzamine alone % Maximal Increase Decrease in the maximal efficacy of Epi due to a decrease in the number of receptors [Agonist], mg/kg

  4. Pharmacological Effects -Phenoxybenxamine • Cardiovascular system Blood pressure Cardiac Effects Organ Blood Flow Capillaries • Central nervous system • Respiratory system

  5. Pharmacological Effects – cont’d • Eye - miosis • GI tract – Increased motility • Urinary bladder – decreased tone in sphincter • Metabolic effects – increased insulin secretion

  6. Adverse effects • Postural hypotension • Tachycardia • Sedation • Nasal stuffiness • Miosis • Impotence (inhibits ejaculation) • Exercise care in hypovolemic patients

  7. Imidazoline derivatives - phentolamine • Many other effects including: • Parasympathomimetic • Increased gastric acid secretion • Cardiac stimulation • Increased secretion from exocrine glands, such as salivary, sweat, lacrimal, pancreatic • Coronary artery disease and peptic ulcer relative contraindication to it.

  8. Alpha-1 selective blockersPrazosin • Less cardiac stimulation since it preserves alpha-2 mediated negative feedback + other mechanisms • Used in congestive heart failure and in hypertension but tolerance develops with time, maybe due to fluid retention. • Adverse effects: First dose phenomenon. • Favorable effect on plasma lipids: increase HDL/LDL ratio

  9. Effect of Adrenaline (ADR) on Blood Pressure and Heart Rate Before and After Prazosin ADR (µg/Kg) HR 0.1 1 10 100 500 BP 500 1 10 100 +PRAZOSIN

  10. Alpha-2 selective blockersYohimbine • Cardiovascular effects – peripheral and central effects • Blocks other receptors also – serotonin, dopamine • Increases ADH release • Enhances sexual activity – aphrodisiac • Potential uses: depression, obesity, NIDDM

  11. Ergot alkaloids • Interact with serotonin and dopamine receptors also • Direct smooth muscle contraction • Structure-activity relationships • Coronary vasoconstriction • Toxicity: GI, vascular insufficiency –ergotism • Use in migraine and post-partum

  12. Therapeutic Uses ofAlpha-Adrenergic Blockers • Hypertension - alpha-1 selective • Conditions associated with increased sympathetic activity – e.g. pheochromocytoma • Hemodynamic shock • Peripheral vascular disease – Raynaud’s • Congestive heart failure • Benign prostatic hyperplasia • Pulmonary hypertension – tolazoline • Yohimbine or intracavernous phentolamine+papaverine for impotence

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