Activation of the Langendorff perfused rat heart under depolarizing conditions

Myocardial activation under depolarized conditions was studied in spontaneously beating Langendorff perfused hearts from albino rats. Depolarization was obtained increasing external potassium concentration in steps (5.4, 7.4, 10, 10.5, 11, 11.5 mM) in the perfusing solution. Left ventricular isovolumic systolic pressure and coronary flow did not change as external potassium increased, but the atrial and ventricular beat rates decreased, the latter showing a larger decline. In the electrocardiogram, the P-R interval increased as a function of external potassium and the amplitude of the QRS complex diminished as its duration increased. The majority of perfused hearts stopped pumping when external potassium was raised to 11.5 mM.