α + β Adrenergic Blockers

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Chapter: Essential pharmacology : Antiadrenergic Drugs (Adrenergic Receptor Antagonists) And Drugs For Glaucoma

Labetalol It is the first adrenergic antagonist capable of blocking both α and β receptors. There are 4 diastereomers of labetalol, each of which has a distinct profile of action on subtypes of α and β receptors.


α + β ADRENERGIC BLOCKERS

 

Labetalol It is the first adrenergic antagonist capable of blocking both α and β receptors. There are 4 diastereomers of labetalol, each of which has a distinct profile of action on subtypes of α and β receptors. The commercial preparation has equal parts of each diastereomer and displays β1 + β2 + α1 blocking as well as weak β2 agonistic activity. The β blocking potency is about 1/3 that of propranolol, while α blocking potency is about 1/10 of phentolamine.

 

Labetalol is 5 times more potent in blocking than α receptors. As such, effects of a low dose resemble those of propranolol alone while at high dose they are like a combination of propranolol and prazosin. Fall in BP (both systolic and diastolic) is due to α1 and β1 blockade as well as β2 agonism (vasodilatation). Relatively high doses reduce both c.o. and t.p.r. Heart rate is unchanged or slightly decreased. In contrast to propranolol, limb blood flow increases with labetalol. It has also been shown to inhibit NA uptake by adrenergic nerve endings.

 

Labetalol is orally effective but undergoes considerable first pass metabolism. It is a moderately potent hypotensive and is especially useful in pheochromocytoma and clonidine withdrawal; can also be used in essential hypertension.

 

Most important side effect is postural hypotension, but this is significant only in some patients. Failure of ejaculation and other side effects of α and β blockers can also occur, but plasma lipid levels are not altered. Rashes and liver damage have been reported.

 

Dose: Start with 50 mg BD, increase to 100–200 mg TDS oral. In hypertensive emergencies 20–40 mg i.v. every 10 min till desired response is obtained.

 

NORMADATE 50, 100, 200 mg tab.

 

Carvedilol It is a β1 + β2 + α1 adrenoceptor blocker; produces vasodilatation due to α1 blockade as well as calcium channel blockade, and has antioxidant property. It has been used in hypertension and is the β blocker especially employed as cardioprotective in CHF. Oral bioavailability of carvedilol is 30%. It is primarily metabolized and has a t½ of 6–8 hrs.

 

CHF: Start with 3.125 mg BD for 2 weeks, if well tolerated gradually increase to max. of 25 mg BD.

 

Hypertension/angina:  6.25 mg BD initially, titrate to max. of 25 mg BD.

 

CARVIL, CARLOC, CARVAS 3.125, 6.25, 12.5, 25 mg tabs; ORICAR 12.5, 25 mg tabs.

 

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