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

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.