concentration is increased in the serum, myocardial contractility is greatly increased; If

more Ca++ inside the cells. As stated already, excess intracellular Ca++ in the cardiac muscle liber means excess contractility. nA and Ad increase contraclilrty ot the heart. ( ii) channel blockers verapamil. nifedipine and diltiazem art well known channel blockers. They (partially) block thoes Slow channels admitting Ca++. Hence they reduce cardiac contractility. These drugs are therefore used in angina pectoris , because reduction of cardiac contractility = reduction of cardiac metabolism = reduction of 02 demand = reduction on the demand on Ihe coronary arteries .These drug also reduce the tone of arteriolar smooth muscles, hence they are popular drugs against hypertension also .On the other hand, digitalis inhibits. The Na+ K+ ATPase —- accumulation of Call within heart muscle cells increased myocardial contractility. Digitalis is used in cases of congestive cardiac failure (CCF), i,g,. where there is weakening of myocardial conlractilriy. The compensatory pause in the phenomenon of exlra systole (fig .5. 2 .10) can now be enplained .Fig 5 ,2 .10 Extrasy stole After a few normal beats, a premature impulse arrives earlier in the diastolic phase and produces a contraction of the ventricles In the meantime, that is. when the systole due to Ihe premature impulse is still on, the regular irn-puise arrives bul as ri has am*ed the venlrices during the refractory penod of the ventricles, dus to Ihe prsmaTurs stimulus, t can produce no affecl The ne*1 beat, which ib due to the newt regular impulse, occurs in proper time therefore the gap between the premature beat and the next regular beat is longer than than between thae two regular beats and is called the 'compensatory pause1 (fig S.2.10) Effects of Ions The important ions to be considered in ihis connection are K++. Ca++ and Na+ K+ Normal concentration of K++ in the entracellular fluid is about 4 in eq/1 A rise of K+ in the ECF produces de-polarization which is nol followed by repolanzation and thus there is loss Of excitability, so thai heart stops in diastole. Before (topping in diastole, the conductivity is senously hampered and heart blocks can appear .A fall of K+ Concentration, on the other hand, unless severe. has usually no remarkable effect In anuria, K+concentration in the plasma begins to rise sharply and sudden cardiac arrest in patients suffering from renal failure is common Ca++. When the Ca++ concentration is increased in the serum, myocardial contractility is greatly increased; If the concentration is sufficiently raised the heart stops in systole, a phenomenon called 'calcium rig or On the other hand, poor availably of Ca++ to the myocardium reduces is cotractile strength In the past, intravenous Ca++ used to be given for various therapeuti