Effects of Cytosolic ATP on Ca 2+ Sparks and SR Ca 2+ Content in Permeabilized Cardiac Myocytes

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Abstract
Abstract— Confocal imaging was used to study the influence of cytosolic ATP on the properties of spontaneous Ca 2+ sparks in permeabilized ventricular myocytes. Cells were perfused with mock intracellular solutions containing fluo 3. Reducing [ATP] to 2+ sparks. In the presence of 20 μmol/L ATP, the amplitude increased by 48.7±10.9%, and the frequency decreased by 77.07±3.8%, relative to control responses obtained at 5 mmol/L ATP. After exposure to a solution containing zero ATP, the frequency of Ca 2+ sparks decreased progressively and approached zero within 90 seconds. As ATP washed out of the cell, the sarcoplasmic reticulum (SR) Ca 2+ content increased, until reaching a maximum after 3 minutes. Subsequent introduction of adenylyl imidodiphosphate precipitated a burst of large-amplitude Ca 2+ sparks. This was accompanied by a rapid decrease in SR Ca 2+ content to 80% to 90% of the steady-state value obtained in the presence of 5 mmol/L ATP. Thereafter, the SR Ca 2+ content declined much more slowly over 5 to 10 minutes. The effects of ATP withdrawal on Ca 2+ sparks may reflect reduced occupancy of the adenine nucleotide site on the SR Ca 2+ channel. These effects may contribute to previously reported changes in SR function during myocardial ischemia and reperfusion, in which ATP depletion and Ca 2+ overload occur.