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After discussing the adrenergic agonists in the previous four lectures now it’s time to discuss the adrenergic antagonists the adrenergic antagonists or also known as adrenergic blockers or sympatholytics are those agents that bind reversibly or irreversibly to adrenergic receptors but do not trigger the usual receptor-mediated intracellular effects they are divided
To two main groups the alpha receptors antagonists and the beta receptors antagonists and each group is further subdivided to non-selective and selective agents in this lecture we’ll discuss the alpha receptors antagonists the lectures pdf will be down in the description so let’s start we already know that alpha 1 receptors are located in the blood vessels when
Activated by endogenous catechol means or agonists it produces vasoconstriction increasing peripheral resistance and blood pressure so we can conclude that blocking alpha adrenoceptors mainly affects blood pressure resulting in decreased peripheral vascular resistance and this induces a reflex tachycardia resulting from the lowered blood pressure the non-selective
Alpha transceptors antagonists are phenoxy benzamine and phentolamine phenoxy benzamine binds covalently to both alpha-1 and alpha-2 receptors so the block is irreversible and non-competitive and the only way the body can overcome the block is to synthesize new adrenoceptors and that may require a day or longer so the actions of lasts for about 24 hours after
The drug is injected a delay of a few hours occurs before a blockade develops on the other hand phantolamine is a reversible competitive blocker for alpha 1 and alpha-2 receptors it lasts for approximately four hours after a single injection both agents block alpha-1 receptors preventing vasoconstriction of peripheral blood vessels by endogenous catecholamines
So decreasing peripheral resistance which provokes a reflex tachycardia they also block presynaptic inhibitory alpha-2 receptors in the heart resulting in more norepinephrine release which stimulates beta 1 receptors on the heart increasing cardiac output and both agents produce what is called epinephrine reversal all alpha adrenergic blockers reverse the alpha
Agonist actions of epinephrine for example we already know from the previous lectures that epinephrine acts on alpha-1 receptors in the blood vessels causing vasoconstriction and also acts on beta-2 in the skeletal muscle’s blood vessels causing vasodilatation so by using the non-selective alpha antagonists the vasoconstrictive action of epinephrine is interrupted
But viscitilation due to beta-2 receptors is not blocked so in the presence of phenoxybenzamine or phentolamine the systemic blood pressure decreases in response to epinephrine and we can conclude that the actions of norepinephrine are not reversed but are diminished because norepinephrine has only alpha one the suppressor action and lacks significant beta agonist
Action on the vasculature and they have no effect on the actions of isoproterenol which is a pure beta agonist phenoxy benzamine is used in the treatment of pheochromocytoma which is a catecholamine secreting tumor of cells derived from the adrenal medulla it is sometimes effective in treating raynaud disease and frostbite phantolamine is used for the short-term
Management of phyochromocytoma it is also used locally to prevent dermal necrosis following extravasation of norepinephrine phantolamine is also useful to treat hypertensive crisis their adverse effects are similar both cause postural hypotension and both induced tachycardia that is mediated by the borrow receptor reflex and by blocking the alpha-2 receptors as we
Mentioned before so they should be used with caution in patients with cardiovascular disease and they are not useful in the treatment of hypertension phenoxy benzamine can cause nasal stuffiness nausea and vomiting it may also inhibit ejaculation let’s now talk about the selective blockers for alpha-1 receptors praise osen terrazosine and doxazosin are selective
Competitive blockers of the alpha-1 receptor so all of these agents decrease peripheral vascular resistance and lower blood pressure they cause minimal changes in cardiac output renal blood flow and glomerular filtration rate so we can conclude that they are used in the treatment of hypertension the first dose of these drugs may produce an exaggerated orthostatic
Hypertensive response this action is known as firsto’s effect it can be minimized by adjusting the first dose to one-third or one-fourth of the normal dose and by giving the drug at bedtime tamsualasin and alphyososin are other selective alpha-1 antagonists indicated for the treatment of benign prostatic hyperplasia they are more selective for alpha-1a receptors in
The prostate and bladder so they decrease tone in the smooth muscle of the bladder neck and prostate improving urine flow with the least effect on blood pressure because it is less selective for alpha 1b receptors found in the blood vessels alpha-1 blockers such as brazosin and doxazosin may cause dizziness a lack of energy nasal congestion headache drowsiness and
Orthostatic hypotension but lesser than that observed with phenoxybenzamine and phentolamine they may also cause inhibition of ejaculation and retrograde ejaculation and the last agent we’ll talk about is a selective competitive alpha 2 blocker it is called yohimbine it is found as a component of the bark of the yohamp tree and has been used as a sexual stimulant
And in the treatment of erectile dysfunction it works at the level of the cns to increase sympathetic outflow to the periphery it is contraindicated in cardiovascular disease psychiatric conditions and renal dysfunction that’s all for this video if that was useful for you please leave like or comment in the upcoming lecture we’ll discuss the beta adrenergic
Antagonists so subscribe if it’s your first time here and keep following us
Transcribed from video
Pharmacology [ANS] 17- Alpha Blockers [ Prazosin, Terazosin, Doxazosin, Tamsulosin and Alfuzosin ] By Medical Videos [ ANIMATED ]
