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Thin trabeculae were dissected from the right ventricle of guinea-pig heart and stimulated to contract isometrically at 0.5 Hz (26 degrees C). Rapid and transient changes of force were obtained by inducing three extra systoles (ES1-3) at 450-ms intervals. The two regular contractions (P1-2) following (ES1-3) were potentiated. Fura-2 salt was microinjected into the preparation to monitor intracellu
