CALCIUM CHANNEL BLOCKERS (Calcium Antagonists)

Calcium has important functions in the body, e.g. regulating contraction of cardiac and smooth muscle and participating the aggregation of platelet. Calcium passes into cells via tiny “channels” in the membrane (called calcium channel).

Calcium-channel blockers (CCB) are a group of medicines which affect the way calcium passes into certain cells. They are used to treat various conditions including high blood pressure, angina, and some arrhythmias, etc.

Ⅰ. Classification
1. IUPHAR(1992)

I: Selectively Block L type Calcium Channels:

Ia: Dihydropyridines:nifedipine、nimoldipine 、nicardipine

Ib:Benzothiazepines:diltiazem

Ic: Phenylalkylamines:verapamil

II: Selectively Block other type Calcium Channels:

III: Non Selective Calcium Channel Blockers:
Flunarizine,Prenylamines
2. According to the applying time of drugs:

First-generation: verapamil、nifedipine、 diltiazem

Second- generation :  felodipine、nimoldipine 、nicardipine et al

Third-generation :  pranidipine、 amlodipine  et al

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Pharmacological actions

1) Effects on heart

a. Negative inotropic effect

b. Negative chronotropic\frequency action and negative dromotropic effect

c. Protective effect on myocytes

2) Effects on Vascular Smooth Muscle

* Relaxation in arterial smooth muscle  → decreased cardiac afterload (aortic pressure)

* Little or no effect on venous beds → no effect on cardiac preload (ventricular filling pressure)

* Specific dihydropyridines may exhibit greater potencies in some special vascular beds (e.g.- nimodipine more selective for cerebral blood vessels)

& Hemodynamic Effects:

All of the clinically-approved CCB:

* Decrease coronary vascular resistance and increase coronary blood flow

* Decrease peripheral resistance via vasodilatation of arteries

* Are no significant effect on venous tone at normal doses

Drug-Specific Effects

Dihydropyridines (e.g. nifedipine, nicardipine, and nimodipine)

*Vasodilatation of arterial resistance vessels

*a reflex increase in sympathetic response

*The overall hemodynamic effect is a drop in blood pressure, an increase in heart rate and contractility, and an increase in cardiac output.

Verapamil

*has greater direct negative effects to heart  than the dihydropyridines

*resulting in a drop in heart rate

*resulting in a lowering of myocardial contractility

Diltiazem

*The hemodynamic effects of diltiazem are intermediate between the dihydropyridines and verapamil

*The drug causes a modest lowering of heart rate and modest decrease in myocardial contractility, both of which are less than verapamil for a given drop in blood pressure

3) Effects on other SM:relax bronchial SM > gastrointestinal SM、ureter and uterus SM

4) other effects:
Antiatherosclerotic effects

Effects on the structure and function of red cells and platelets.

Effects on the renal function

Clinical Uses

(1) The primary indications for the Calcium channel blockers are

* Hypertension * Angina * Arrhythmias

(2)Cerebralvascular disease (nimodipine, Flunarizine): cerebral vasospasm,cerebral embolism, cerebral ischemia.

(3) Others: Radynau’s disease; Bronchial asthma; Migraine; Atherosclerosis

Toxicities and Side Effects

* The calcium channel blockers are generally well-tolerated .

* The most common side effects are due to excessive vasodilatation (i.e.- dizziness, hypotension, headache, flushing etc.) , the other side effect is constipation.

* Serious toxic effects

bradycardia, AV block, transient asystole, exacerbation of heart failure are rare and usually occur under specific conditions. Patients with moderate to severe ventricular dysfunction, SA node or AV conduction disturbances, and systolic blood pressures below 90 mm Hg should not be treated with verapamil or diltiazem.

 

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Posted on มกราคม 15, 2013, in บทความ. Bookmark the permalink. ใส่ความเห็น.

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