INTERACTIONS BETWEEN CALCIUM WAVES AND ACTION POTENTIAL-INDUCED CALCIUM TRANSIENTS IN GUINEA-PIG MYOCYTES

In isolated cardiac muscle, spontaneous Ca2+ release from the sarcopla smic reticulum (SR) occurs and is propagated as a wave by a regenerati ve 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 w ere 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, intera ctions between fluorescence transients due to propagating waves and ac tion potential-induced fluorescence transients were observed. In this study, the action potential-induced fluorescence transients could be d etected 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 chan ge whenever the preceding waves were propagating. These results show t hat the interaction between the action potential-induced Ca2+ release and the calcium wave-induced Ca2+ release from the SR have the followi ng characteristics: (1) For the action potential-induced Ca2+ release no absolute refractory period was observed 33msec after the calcium wa ve. This suggests that the calcium waves can be reset by the action po tential. (2) Regardless of whether the two release mechanisms are diff erent, both share a common compartment of Ca2+ storage.