[K+ loss by the myocardial mitochondria in fibrillation due to enhanced intracellular intake of Ca2+].

Ventricular fibrillation developed in an isolated perfused rat heart one minute after Na+ concentration was reduced in the solution, with homogenate and mitochondrial amount of 45Ca increasing. Subsequent perfusion with a solution of normal composition rapidly resulted in spontaneous defibrillation of the heart and the recovery of baseline radioactivity in the homogenate and mitochondria. As mitochondrial Ca2+ increased, K+ decreased simultaneously. It is suggested that intensive myocardial K+ loss at ventricular fibrillation is due to K+ withdrawal from the mitochondria as they absorb Ca2+ coming in excess to myocardial cells.