Activation of the langendorff perfused rat heart under depolarizing conditions

Myocardial activation under depolarized conditions was studied in spontaneoustly beating Langendorff perfused hearts from albino rats. Depolarization was obtained incrasing external potassium concentration in steps (5.4, 7.4, 10, 10.5, 11, 11.5 mM) in the perfusisng 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 ORS complex diminished as its duration invreased. The mamjority of perfused hearts stopped punping when external potassium was raised to 11.5 mM