Effect of heart rate and ouabain on action of calcium on atrial contractions

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Abstract
The influence of heart rate and ouabain on the changes in rabbit atrial contractile tension induced by a variation of extracellular Ca concentration was studied. When the Ca concentration is lowered from normal to one-sixth normal (2.4-0.4 m[image]), contractile tension decline exponentially in time in one phase only Tat 100 beats/min, t1/2 = 73 [plus or minus]4.3 sec). An inverse relation between heart rate and t 1/2 was observed. However, the cumulative number of beats for the various t1/2 values were relatively constant. Ouabain (5 x 10-7 [image]) accelerated both the rate of decline and restoration of tension when the Ca was first lowered, then restored to normal. Norepinephrine (5 x 10-6 [image]) markedly delayed the rate of tension decline in Ca-deficient medium. Rate of tension change induced by variations in the extracellular Ca is limited by the rate of which Ca leaves or enters a contraction-dependent pool in the myocardium. Ouabain acts by accelerating exit and entry rates in this tissue compartment.