Alpha blocking drugs l Antiadrenergic drugs l Study Pharma l

 Antiadrenergic drugs:

Antiadrenergic drugs are those drugs that inhibit the adrenergic response mediated by alpha or beta adrenergic receptors by blocking adrenaline binding to their specific alpha or beta receptors.

On the basis of receptors, antiadrenergic drugs classified into:-

1. Alpha blocking drugs or Alpha blockers

2. Beta blocking drugs or Beta blockers

Alpha blocking drugs:

These drugs inhibits adrenergic response mediated through alpha adrenergic receptors by blocking the binding of adrenaline to alpha adrenergic receptors without affecting those mediated through beta receptors.

Pharmacological action of Alpha blockers:

Blood vessels:

- Vasodilation of blood vessels occur due to blocked of alpha agonist binding site by alpha blockers or antagonist.

- Peripheral resistance reduced

- Venous return and cardiac output reduced

- Fall in BP (marked hypotension occur)

- Reflex tachycardia occur due to fall in arterial pressure and increase in noradrenaline release from cardiac sympathetic neurons due to blocked of presynaptic alpha 2 receptors.

- Nasal stuffiness and miosis occur due to blocked of alpha receptors.

GIT:

- Intestinal motility increases because of inhibition of partial sympathetic response mediated by alpha receptors.

- Renal blood flow and GFR is reduced

- More reabsorption of sodium ions and water occur

- Reflex increase in renin amount which mediated through beta 2 receptors

Smooth muscles:

- Tone of bladder trigone, sphincter and prostate is reduced which mediates urination (person who suffer from urinary retention and Benign hypertrophy of prostate treated).

Classification of Alpha blocking drugs:

Phenoxybenzamine:

- It is a non-equilibrium type alpha blockers or adrenergic antagonists.

- It is lipid soluble.

- Last up to 3-4 days till fresh receptors are formed.

- Partial blocked the 5-HT, histaminergic and muscarinic receptors but not affected beta adrenergic receptors.

- Cause fall in BP, postural hypotension may be seen, produce CNS stimulation cause vomiting and nausea by activating CTZ when injected through i.v. route.

- Oral administration cause depression, tiredness, lethargy like condition.

Natural and hydrogenated ergot alkaloids:

- They are equilibrium type  of non-selective alpha blockers or adrenergic antagonists.

- They are more potent alpha blockers because they produce long lasting effect

Phentolamine:

- It is a non-selective alpha blocker categories under Imidazoline.

- It is rapid acting and short duration of action.

- Because of non-selective, it blocks alpha 1 and alpha 2 receptors.

- Noradrenaline release increases due to blocked of alpha 2 receptors and cause tachycardia.

Prazosin:

- It is a selective alpha 1receptors blockers.

- It only block alpha 1 receptors, by this noradrenaline release not occur because it not block the action that mediated by alpha 2 receptors, means no tachycardia occur.

- Cause vasodilation, postural hypotension is less marked, dizziness and fainting as first dose effect,

Uses:

- Pheochromocytoma:

It is a tumour of adrenal medulla cells which release excess amount of catecholamines. To stop the secretion of catecholamines gives alpha blockers which inhibits the release.

- Hypertension:

Alpha blockers dilates the blood vessels which reduce heart rate, BP and ultimately hypotension occur.

- Benign hypertrophy prostate (BHP):

It is a condition in which person unable to urinate properly. At this condition, alpha blockers given to person who suffer from BHP that produce to reduce the tone of bladder trigone, sphincter and prostate and promote urination.

- Peripheral vascular disease:

Alpha blockers given which cause vasodilation and reduce peripheral resistance, ultimately peripheral vascular disease treated.

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