Cardiac Muscles and Calcium Channel Blockers

The topics in this post include structure of cardiac muscles, mechanism of muscles contraction, as well as different type of CCB and their uses.

A.Structure of Cardiac Muscles 

Figure 1: structure of cardiac muscles (source)

Figure 2: The diagram shows the thin and thick filaments.

Sarcomere is the contractile unit of the cardiac muscle. It contains thin filament (actin), thick filament (myosin), troponin and tropomysin. If you have difficulties in differentiating thin and thick filament (just like me), just remember "actin" has a sound like "thin".

Figure 3: this diagram shows the position of actin, troponin and tropomyosin. There is calcium ion binding site on troponin which regulate the movement of tropomyosin. (source)

Actin contains actin binding sites which allow myosin head to bind to carry out sliding mechanism. However, they are covered by tropomysin. Troponin regulates the movement of tropomysin through the binding of calcium ion. 

b Mechanism of  Muscle Contraction

Figure 4: the diagram showed the flowchart of cardiac muscle contraction. Note that the troponin complex here means tropomysin. Step no.4 is shown in Figure 3.

Figure 5: Sliding mechanism(source)

C. Calcium Channel Blockers

    Calcium channel blockers (CCB) can be considered as a vasodilator. CCB interfere with the inward displacement of calcium ions through the slow channels of active cell membranes. They influence the myocardial cells and the cells of vascular smooth muscle. Thus, they may reduce 

(a) myocardial contractility 

(b) the formation and propagation of electrical impulses within the heart

 (c) coronary or systemic vascular tone may be diminished.    [Reference:BNF (May)]

1. Type of CCB:
(i) Dihydropyridines 

• amlodipine

• felodipine

• isradipine

• nicardipine

• nifedipine

• nimodipine

• nitrendipine

(ii) Non-dihydropyridines 
-Diltiazem
-Verapamil

CCBs differ in their predilection for the various possible sites of action and, therefore, their therapeutic effects are disparate.

(i) Dihydropyridines 
- most vaso-selective, least cardiac effect

 The side effects include:
-flushing and headache(associated with vasodilation)
-ankle swelling (which may respond only partially to diuretics) are common.

(ii)non-dihydropyridines

So all type of CCB can be used to treat hypertension. Verapamil has the highest cardiac depressant effect, followed by diltiazem and nifedipine.
V,D,N= Very Dangerous Neighbour

Reference: Dr.Subrat notes on hypertension & British National formulary (May).