Ca+ waves have been implicated in Ca2+ overload-induced cardiac arrhythmias. To deepen understanding of the behavior of Ca2+ waves in a multicellular system, consecutive two-dimensional Ca2+ images were obtained with a confocal microscope from surface cells of guinea pig ventricular papillary muscles loaded with fluo 3 or rhod 2. In intact muscles, no Ca2+ waves were detected under the resting condition, whereas they were frequently observed during the rest immediately after high-frequency stimulations where cytoplasmic Ca2+ concentration and Ca2+ stored in the sarcoplasmic reticulum (SR) were gradually decreasing. The intervals of Ca2+ waves increased as they occurred later, their amplitudes and velocities remaining unchanged. A SERCA inhibitor reversibly prolonged the wave intervals. In Na+-free/Ca2+-free medium where neither Ca2+ influx nor Na+/Ca2+ exchange took place, recurrent Ca2+ waves emerged at constant intervals in each cell. These results are consistent with the conclusion that the loading level of the SR is critical for induction of Ca2+ waves. Each cell independently exhibited its own regular rhythm of Ca2+ wave with a distinct interval. These waves propagated in either direction along the longitudinal axis within a muscle cell, but seldom beyond the cell boundary. In contrast, in partially damaged muscles that showed spontaneous Ca2+ waves at rest in normal Krebs solution, their propagation often was unidirectional, decreasing in frequency. In these cases, however, Ca2+ waves rarely moved beyond the cellular boundary. The gradient of the cytoplasmic Ca2+ concentration was suggested to be the cause of the one-way propagation.
