Phenoxybenzamine is an irreversible alpha blocker used in the treatment of hypertension. It has a slower onset and a longer lasting effect compared with other alpha blockers.

Pharmocology

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Phenoxybenzamine is used as an anti-hypertensive due to its efficacy in reducing the vasoconstriction caused by norepinephrine. Drugs such as phenoxybenzamine block the α1 adrenoreceptors on the postsynaptic membrane and thus prevent the noradrenaline from bonding with the α1 adrenoreceptors. The effect of this is that the sympathetic nervous system is inhibited leading to a greater effect from the parasympathetic. This results in vasodilation, pupil constriction, an increase in GI tract motility and secretions, glycogen synthesis and impotence. Such responses inhibit the “fight or flight” mechanism.

References

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Day, M.D. (1979). Autonomic Pharmacology Experimental and Clinical Aspects. Churchill Livingstone.

Bullock, S. Galbraith, A. Hunt, B. Manias, E. and Richards, A. (1999). Fundamentals of Pharmacology. Pearson Education Limited publishing as Prentice Hall.