α + β Adrenergic Blockers
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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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