Phentolamine is non selective reversible alpha blocker, histamine agonist (H1, H2) and muscarinic agonist. It also has inhibitory effects at serotonin receptors.
Pharmacokinetics:
Oral absorption is unpredictable. Peak plasma levels are observed after 1 hour. Half life is 5-7 hours.
Uses:
Used in pheocytochroma.
Adverse Effects:
Its adverse affects include cardiac stimulation, postural hypertension, reflex tachycardia, headache, arrhythmias, diarrhea and palpitations.
Phenoxy benzamine:
Irreversible alpha blocker, forms covalent bonds. Although being non selective, it is more selective for alpha 1 than alpha 2 receptors. It forms ethylene ammonium metabolite, making strong bond with alpha receptors.
Pharmacokinetics:
It has a very long half life of about 14-48 hours. It is lipid soluble and can cross the blood brain barrier to cause sedation, fatigue, nausea, decreased TPR, postural hypertension, reflex tachycardia. It has lesser intensity than phentolamine. It also inhibits the reuptake of nor adrenaline.
Uses:
It has a role in management of pheochromocytoma.
| Phenoxybenzamine | Prazosin |
| Irreversible antagonism | Reversible antagonism |
| Alpha 1 and 2 selective | Alpha 1 selective blockers |
| Decrease TPR and B.P, produce vasodilatation and reflex tachycardia | Veins are less affected, less reflex tachycardia and less decrease in TPR and B.P |
| May cause sedation, nausea and fatigue | May cause 1st dose phenomenon |
| Used in pheochromocytoma | Used in HTN and BPH |
| Can cross blood brain barrier (Antihistamine in nature) | Cannot cross blood brain barrier |
Prozasin:
Selective alpha 1 blocker, causing decrease in TPR and venous return. It has no effects on alpha 2 receptors.
Pharmacokinetics:
It has a half life of 3 hours. Its bioavailability is 50%.
Adverse Effects:
Adverse effects include:
- Postural hypotension
- Causes first dose response
When administered, causes postural hypotension and the person may collapse. To cope this, first a small dose is administered, which is increased very slowly. It is preferably administered at night, and the patient is advised to stay in bed in lying position. Sudden withdrawal is avoided and other hypertensives are withdrawn. Otherwise severe hypertension might occur. The patient is strictly warned about the side effects.
| Phenoxybenzamine | Prazosin |
| Irreversible antagonism | Reversible antagonism |
| Alpha 1 and 2 selective | Alpha 1 selective blockers |
| Decrease TPR and B.P, produce vasodilatation and reflex tachycardia | Veins are less affected, less reflex tachycardia and less decrease in TPR and B.P |
| May cause sedation, nausea and fatigue | May cause 1st dose phenomenon |
| Used in pheochromocytoma | Used in HTN and BPH |
| Can cross blood brain barrier (Antihistamine in nature) | Cannot cross blood brain barrier |
Terazosin, Tamsulosin and Yohimbine
Terazosin
Terazosin is useful in prostate diseases.
Tamsulosin
Selective alpha 1 a blocker, useful in benign prostatic hypertrophy, especially in those not having hypertension.
Yohimbine
Selective alpha 2 blocker, obtained from plant source. Its therapeutic importance is not established. Experiments are being done regarding its use in clonidine withdrawal, in which alpha 2 receptors are activated.
In diabetes mellitus, insulin is inhibited. Role is also being investigated in relief in depressive syndromes.