Calcium-induced calcium release activates contraction in intact cardiac cells.

The "caged" calcium chelator Nitr-5 was incorporated into isolated rat ventricular myocytes. Brief illumination with ultra-violet light made the cell twitch. The light-induced twitch was inhibited by ryanodine (1-10/microM) suggesting that it resulted from calcium-induced release of calcium from the sarcoplasmic reticulum. Inhibition of the Ca current (Ni, 10 mM) abolished the electrically stimulated twitch but did not inhibit the light-induced twitch. These results provide direct evidence for the importance of Ca-induced Ca release in excitation-contraction coupling in the heart.