What is Alpha Blockers? Side effects of alpha blockers? Alpha blockers

     

 ALPHA ADRENERGIC RECEPTOR  BLOCKING AGENTS

              (ALPHA BLOCKERS)

Drugs with diverse chemical structures block alpha adrenergic receptors and inhibit their responses.The blockade is reversible except in case of  phenoxybenzamine which causes irreversible blockade  due to its covalent biding to alpha receptors.
  These drugs differ regarding their capability of blocking the two subtypes of alpha receptors i.e. postsynaptic alpha 1 receptors and presynaptic alpha 2 receptors. Prazosin is a selectoive blocker of postsynaptic alpha 1 receptors.Phenoxybenzamine is more potent in blocking postsynaptic alpha 1 receptors than in blocking presynaptic alpha 2 receptors. Phentolamine and tolazoline are nonselective blockers of alpha receptors. On the other hand yohimbine is a selective alpha 2 blocker.

CLASSIFICATION OF ALPHA BLOCKERS:

1. CHEMICAL CLASSIFICATION:

• BETA-Halokylamines

           Phenoxybenzamine
           Dibenamine

• Substituted Imidazolines

           Phentolamine

           Tolazoline

• Phenoxyalkylamines

          Thyroxamine

• Ergot alkaloids

           Ergotamine

           Dihydroergotamine

• Dibenzapines

           Azapetine

• Benzodioxans

           Piperoxan

• Indolealkylamines

           Yohimbine

• Phenothiazines

           Chlorpromazine

• Miscellaneous

           Prazosin

           Indoramine

           Labetalol

   
      2.  CLASSIFICATION BASED  ON DURATION OF ACTION:

• Reversible 

           Phentolamine

           Tolazoline

           Prazosin

• Irreversible

           Phenoxybenzamine

           Dibenamine

      3..CLASSIFICATION BASED ON SELECTIVITY OF ACTION:

• Selective alpha 1 Postsynaptic blocking agent

           Prazosin

• Predominantly aplha 1 Postsynaptic blocking agent

           Phenoxybenzamine

• Selective alpha 1 Presynaptic blocking agent

          Yohimbine

• Non-selective( Blocks both alpha 1 and alpha 2)

          Phentolamine

 PHARMACOLOGICAL ACTIONS OF ALPHA BLOCKERS

    These drugs bind to alpha adrenergic receptors and interfere with the capacity of sympathomimetic amines to initiate action at these sites.They prevent the response of effector organs to adrenaline,nonadrenaline and other sympathomimetic amines whether released in the body or injected. Circulating catecholamines are antagonized more readily than are the effects of sympathetic nerve stimulation. Phenoxybenzamine binds covalently to the alpha receptors and produce irreversible blockade. Other drugs bind reversibly and antagonize the action of sympathomimetic amines competitively.

Vasodilation:

  The principal action of all alpha blackers is to produce peripheral vasodilation, with the exception of ergotamine which possesses a direct vasoconstrictor action.

Vasomotor Reversal:

   Alpha blockers reverse the pressor effect of adrenaline due to the unmasking of beta effects producing vasodilation of skeletal muscle vessels and hypotension. These drugs abolish the pressor effect of nonadrenaline which possesses mainly alpha effects.

Miosis:

  These drugs constrict pupil of eyes by blocking the action of fibres of iris.

THERAPEUTIC USES OF ALPHA BLOCKERS:

Peripheral Vascular Spastic Disease:

   With the exception of the ergot alkaloid,alpha blockers have been used in the treatment of peripheral vascular spastic diseases for such conditions as thromboangitis obliterans(Buerger's disease) and Raynaud's syndrome. It is assumed that sympathetic hyperactivity contributes in part to these conditions and that this component is eliminated by alpha receptors blockade. Alpha blockers are used for treatment of chillblains and intermittent claudication.

Phaeochromocytoma:

   Alpha blockers are used in the investigation and in the preoperative treatment of phaechoromocytoma which is a rare hyperfunctioning tumour of adrenal medulla. It is composed of chromaffin tissue and secreates large quamtutues of adenaline and nonadrenaline, alpha adrenergic blockade transiently lowers the elevated arterial pressure that is characteristic of this tumour.

Hypertension:

  Treatment of hypertension with alpha blockers except prazosin has been rather disappointing due to reflex tachycardia and palpitation produced by them. Usefulness of prazosin in hypertension may be due to its lack of potency in inhibiting presynaptic alpha 2 receptors. Phenoxybenzamine and phentolamine have been used successfully to control acute hypertensive episodes due to symparhomimetics,amd to certain foods and drugs in the presence of MAO inhibition.

Autonomic Hyperreflexia:

   It follows high spinal cord transection and is associated with paroxysmal elevation in blood pressure. These pressor episodes and associated signs ans symptoms are well controlled by relatively small oral doses of phenoxybenzamine,

Glaucoma:

    Alpha blockers may be use in glaucoma.

Migraine:

   Ergotamine is of particular value in migraine due to its direct vasoconstrictor effects.

Shock:

 In certain types of shock when reflex neurogenic vasoconstriction due to hyperactivity of compensatory mechanisms is a prominent feature, the judicial use of alpha blockers can increase local blood flow in important areas, kidney and mesenteric bed, where local anoxia is relieved and recovery facilitated. These drugs should not be given unless the central venous pressure has been elevated by fluid administration without an adequate circulatory response. However use of dopamine is preferred in such situations.

ADVERSE EFFECTS OF ALPHA BLOCKERS:

• Postural Hypotension
• Tachycardia
• Nasal Stuffiness
• Sedation
• Diarrhoea
• Failure of ejaculation

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