Intrinsic Tension and Shortening Velocity of Intact Segment without Damaged Cells in Bullfrog Cardiac Muscle
The tension and length change of the bullfrog atrial trabeculae muscle were measured under control of the central segment length. Damaged cells located at both ends of the muscle preparation were avoided in the segment. Two pieces of thin black tape were placed circumferentially on the central portion of the muscle preparation 5 to 7 mm apart from each other. The movement of each piece of tape was recorded by a photoelectrical device and the distance between the two pieces of tape was measured through a differential amplifier. The output voltage of the amplifier was fed back to an ammeter, so that the length of the central segment could be kept constant during the contraction. Even in the ordinary isometric contraction, the central intact segment shortened by around 10% in compensation for stretching the peripheral damaged segments. The ratio of the segment isometric tension to the muscle isometric tension reached 1.5 to 2.0. The shortening velocity under nominally 0 load measured from the intact segment was 120% of the one measured from a whole muscle.