Spontaneous calcium waves in isolated rat cardiomyocytes were investigated by confocal laser scanning microscopy using the fluorescent Ca(2+)-indicator fluo-4 AM. With increasing calcium overload propagation velocities reinforced. The calcium wavespeed was significantly diminished by drugs which interfere with the calcium uptake of both the sarcoplasmic reticulum (SR) and mitochondria, respectively. Stepwise addition of thapsigargin, a highly specific inhibitor of SERCA, decreased the wavespeed and allowed the determination of flux control coefficients which were found to be increasing from 0.15-0.75 in dependence on calcium overload. Kd was estimated to be between 0.4 and 0.6 nM TG. At 5 mM TG wavespeed was significantly reduced by almost 50%. Spontaneous calcium waves did not occur in bathing solutions with more than 20 nM thapsigargin. Calcium wave velocity was also reduced in the presence of the oxygen-bridged dinuclear ruthenium amine complex RU 360 which specifically blocks the mitochondrial Ca2+ uptake. The observed effects are likely due to a reduction of the ryanodine receptor's open probability. It is suggested that the intracellular Ca2+ signaling depends on both SR lumenal and cytosolic calcium concentration.