Effects of Cytosolic ATP on Ca Sparks and SR Ca Content in Permeabilized Cardiac Myocytes

Confocal imaging was used to study the influence of cytosolic ATP on the properties of spontaneous Ca sparks in permeabilized ventricular myocytes. Cells were perfused with mock intracellular solutions containing fluo 3. Reducing [ATP] to ,0.5 mmol/L decreased the frequency but increased the amplitude of spontaneous Ca sparks. In the presence of 20 mmol/L ATP, the amplitude increased by 48.7610.9%, and the frequency decreased by 77.0763.8%, relative to control responses obtained at 5 mmol/L ATP. After exposure to a solution containing zero ATP, the frequency of Ca sparks decreased progressively and approached zero within 90 seconds. As ATP washed out of the cell, the sarcoplasmic reticulum (SR) Ca content increased, until reaching a maximum after 3 minutes. Subsequent introduction of adenylyl imidodiphosphate precipitated a burst of large-amplitude Ca sparks. This was accompanied by a rapid decrease in SR Ca content to 80% to 90% of the steady-state value obtained in the presence of 5 mmol/L ATP. Thereafter, the SR Ca content declined much more slowly over 5 to 10 minutes. The effects of ATP withdrawal on Ca sparks may reflect reduced occupancy of the adenine nucleotide site on the SR Ca channel. These effects may contribute to previously reported changes in SR function during myocardial ischemia and reperfusion, in which ATP depletion and Ca overload occur. (Circ Res. 2001;89:526-533.)