Sarcomere length-tension relations of frog skinned muscle fibres during calcium activation at short lengths.

1. Single twitch fibres were isolated from anterior tibial muscles of the frog, Rana pipiens. The relationship between sarcomere length and steady tetanic tension at 5 degrees C was obtained from these living fibres in the range of sarcomere lengths between about 2.2 and 1.3 microns . These fibres were then either mechanically or chemically skinned. 2. Segments were cut from the skinned fibres and mounted in an experimental chamber using a technique designed to minimize segment compliance at the points of attachment. A piece approximately 1 mm in length remained exposed to the bathing solution. 3. The segments were photographed through a light microscope at magnifications of about 460 or 110 X during activation and relaxation, so that the sarcomere lengths could be determined from a part or the whole of the segment. Activations were done with solutions of pCa either 5.49 or 6.09 and at a temperature of 5 degrees C. Fibre segments which developed striation pattern irregularities during contraction were rejected. 4. The sarcomere length-tension relation obtained from these segments in the sarcomere length range 1.3-2.2 microns was similar to that obtained from the same fibres while still living. The results were similar at the two values of pCa used. 5. These results do not support the view that sarcomere length dependent variation in the amount of calcium which is released during tetanic stimulation is a major determinant of the form of the length-tension relation in living muscle fibres at sarcomere lengths less than about 2.0 microns.