Interactions between calcium waves and action potential-induced calcium transients in guinea pig myocytes

In isolated cardiac muscle, spontaneous Ca²⁺ release from the sarcoplasmic reticulum (SR) occurs and is propagated as a wave by a regenerative Ca²⁺-induced Ca²⁺ release mechanism. We have already reported that this wave is followed by a refractory period. The aim of this study is to investigate whether such a refractory period could also inactivate Ca²⁺ release from the SR triggered by an action potential. Myocytes were enzymatically isolated from guinea pig ventricles and loaded with acetoxymethylester form of fura-2 (fura-2 AM). The membrane potential was recorded with a conventional microelectrode technique, and spatio-temporal changes in fura-2 fluorescence were recorded using a digital TV system. After perfusion with potassium-free Tyrode solution, interactions between fluorescence transients due to propagating waves and action potential-induced fluorescence transients were observed. In this study, the action potentialinduced fluorescence transients could be detected in the next video frame after the propagation of the waves and showed gradual restitution of the transients. In addition, the sum of the fluorescence transients triggered by an action potential and the fluorescence transients due to the waves did not show significant change whenever the preceding waves were propagating. These results show that the interaction between the action potential-induced Ca²⁺ release and the calcium wave-induced Ca²⁺ release from the SR have the following characteristics: (1) For the action potentialinduced Ca²⁺ release, no absolute refractory period was observed 33 msec after the calcium wave. This suggests that the calcium waves can be reset by the action potential. (2) Regardless of whether the two release mechanisms are different, both share a common compartment of Ca²⁺ storage.