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I. α-Adrenergic Blockers

I. α-Adrenergic blockers:1. Non-selective α-blockers:

β-Haloalkylamine (Phenoxybenzamine)

Imidazolines (Phentolamine & Tolazoline)

Ergot alkaloids (e.g., Ergotamine)

2. Selective α1-blockers: Prazosin, terazosin, doxazosin &

tamsulosin3. Selective α2-blockers

Yohimbine2

• General properties of α-blockers:

1. They act by reversible competitive antagonism with the exception of phenoxybenzamine.

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2. They act non-selectively on both α1-

and α2-adrenoceptors except prazosin (doxazosin, terazosin and tamsulosin) and indoramin that act specifically on α1 receptors and yohimbine which acts selectively on α2 receptors.

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3.The use of non-selective α-adrenoceptor blockers to treat hypertension (anti α1 vasodilatation of blood vessels) is hampered by a reflex tachycardia aided by α2 inhibition ( release of NA).

The use of selective α1-blockers is

beneficial since they do not induce compensatory tachycardia. 5

4. With the exception of yohimbine, α-adrenoceptor blockers reverse the pressor effect of adrenaline (adrenaline reversal), antagonize partially that of NA while that of ephedrine, phenylephrine and methoxamine is abolished.

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