Phenoxybenzamine makes covalent bond with alpha receptors, thus dissociation is slow. Phenoxybenzamine forms ethylene ammonium which binds alpha receptors, forming strong bonds. Pharmacological Actions 1. CVS a.
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Phenoxybenzamine makes covalent bond with alpha receptors, thus dissociation is slow. Phenoxybenzamine forms ethylene ammonium which binds alpha receptors, forming strong bonds.
Pharmacological Actions 1. CVS a. Blood Pressure Tone of smooth muscles of both arteriolar and venous blood vessels is maintained by alpha receptors. The blockers act to cause vasodilatation, producing decreased TPR and decreased blood pressure. Vasoreversal phenomenon This was first discovered by Dale in , thus is also called Dale vasomotor reversal phenomenon.
When adrenaline is administered it acts on both alpha and beta receptors, causing increased TPR and increased blood pressure both systolic and diastolic. When alpha blockers are given, they antagonize the effects of alpha receptors, causing decrease in systolic blood pressure. Adrenaline effect is reversed by administration of alpha blockers. This has no therapeutic importance. Postural Hypotension Sympathetic activity is maintained by alpha 1. When blocked, a standing person has pooling of blood in the periphery.
Due to the fact that alpha 1 receptors are blocked, vasodilatation occurs and blood flow to periphery is increased, while venous return is decreased. The constriction of arterioles further causes peripheral filling of blood and peripheral hypotension.
Reflex tachycardia Alpha 2 blockers non-selective alpha blockers are seen more to cause reflex tachycardia. The negative feedback mechanism in NE release by alpha 2 receptors presynaptic causes decreased noradrenaline. When less is released, an increase in release is observed. When alpha 2 receptors are blocked, this negative feedback is inhibited. Uninhibited release of NE has effects on beta receptors beta 1 , causing tachycardia and arrhythmias.
Eye —Misois Alpha 1 receptors are found in the papillary dilator muscles while constrictors are acted upon by M3. Due to alpha blockers, the constrictors are unopposed and cause miosis. This has beneficial role in glaucoma, but alpha blockers have no role in treatment of glaucoma. Nasal Stuffiness Vasodilatation within mucous membrane of nasopharynx occurs, causing nasal stuffiness.
Bladder Alpha 1 receptors are also found in the base of bladder and neck of prostate. A decrease in pressure is observed leading to decreased mechanical resistance to flow of urine. This is important in the treatment of benign prostatic hyperplasia.
Useful clinically in urinary retention due to BPH. GIT When alpha 1 receptors are blocked, sphincters are opened, leading to increased motility, which may cause diarrhea and vomiting. Vas Deferens Decrease contraction of vas deferens smooth muscles leading to difficulty in ejaculation.
Therapeutic Uses 1. Antihypertensive Due to blockage of alpha 1 receptors, there is decreased vasoconstriction, leading to decreased HTN. In mild to moderate hypertension, Prazosin short acting is used. Others include Doxazosin, terazosin which are longer acting. Effective hypertensive emergency must be coped with.
Labetolol is more efficient as is more selective, Urapidil may be used as well Alpha blockers are not the first line of defense. Indication is also seen in sudden withdrawal of Clonidine. Alpha 2 selective drugs are used as sudden hypertension might be observed. Benign Prostatic Hypertrophy This occurs in elderly male. An obstruction to the flow of urine occurs resulting in decreased smooth muscle mass.
Alpha 1 stimulation leads to further vasoconstriction and decrease in flow of urine. This results in nocrurine and dribbling of urine. Surgical operation is recommended. In some cased drug therapy may prove to be beneficial. Prazosin and Terazosin selective alpha 1 antagonists like drugs are given which may cause tone reversal. Newer agents like Tamsulosin alpha 1 a are highly effective, especially in non-hypertensive patients. Finasteride may be given in combination with dexazosin to cause the reversal of smooth muscle mass in prostate.
Buy Finasteride Propecia Online. Local Vasoconstrictor Excess To enhance the absorbance at site of application, vasoconstrictors adrenaline are added to local anesthetic. If given in excess, to reverse the effects, alpha blockers are given to block the vasoconstrictor effects.
Pheochromocytoma Tumor of adrenal medulla in which chromaffin cells are involved, causing increase in adrenaline and noradrenaline. This results in tachycardia, palpitations, headache, excessive sweating and increased sympathetic activity. The condition can be diagnosed by 3 hydroxy 4 methoxymandelic acid or by the presence of metabolites in urine or by using radio-isotopes, CT scan or MRI.
Alpha blockers are effective. Phentolamine is used intraoperatively best treatment surgery to reverse the increased blood pressure and tachycardia by drug therapy. More advanced techniques are available now to counter the rise in blood pressure. Beta blockers are always administered before alpha receptor blockers have been instituted to reverse cardiac effects of excessive catecholamines. Due to blockage of negative feedback mechanism because of blockage of alpha 2 receptors, noradrenaline may affect heart to produce tachycardia.
Beta blockers prevent NE effects on heart and tachycardia. This is to counter effects of alpha blockers. Labetolol non-selective alpha beta blocker is also effective in pheocytochroma. Metyrosine decreases the synthesis of adrenaline and noradrenaline. It is effective in metastatic and refractory cases. We give metyrosine with phenoxy benzamine. This is not considered appropriate for operation and refractory cases.
Congestive Cardiac Failure Prazosin and congeners are used. They decrease the after load and preload. Erectile Dysfunction Alpha blockers have no role in erectile dysfunction. Peripheral Vascular Spastic Disease a. Intermittent claudication due to spasm of muscles. Phentolamine can cause vasodilatation. Migraine Stress induced vasoconstriction leads to hypoxia, causing the smooth muscles of vessels to relax resulting in vasodilatation.
Thus blood pressure is not maintained and stretch of vessels in region of trigeminal nerve supply lead to migraine. Nowadays, ergotamine is given. Shock If fluid replacement fails, alpha blockers are given, thus decreased vasoconstriction leads to increased cardiac output and blood moves from pulmonary to systemic circulation and from extra vascular to vascular compartment.
Autonomic Hyper-reflexia During spinal cord transaction, there is increased vasoconstriction, so alpha blockers are given. Reflex tachycardia non selective alpha blockers, selective alpha 1 blockers are less prominent having little or no effect on alpha 2 receptors Nasal stuffiness Sedation esp.
DALES VASOMOTOR REVERSAL PHENOMENON PDF
Vasomotor reversal of dale We said that after administration of a alpha blocker, we saw a fall in blood pressure. I drew, revised and revised for remembering life cycles. And also, could you please share about cestodes, trematodes and nematodes? B blockers are dalee used to treat hypertension.
DALES VASOMOTOR REVERSAL PHENOMENON PDF