Length-tension relation of cat heart muscle studied by a segment-control method

We studied the effects of the damaged ends on the length-tension relationship of the cat right ventricular papillary muscle during twitch contraction (30/min) using a newly developed segment-control system. The segment length (SL), defined as the distance between two thin black films attached to the central surface of the muscle with silicone grease, was measured by a Hamamatsu Photonics Width Analyzer (C1170) and Camera (C1000) and controlled with a servoactuator. The marker distance was determined to be either the central 24%-30% (short marker distance) or 47%-56% (long marker distance) of the muscle length (ML) at which the maximum developed twitch tension was observed (MLmax). The muscle was lengthened and shortened in a triangular fashion under both ML and SL isometric twitch modes over a period of 250s. The relationship between ML or SL and developed tension (DT, total minus resting tension) was obtained from the twitch tension data during the lengthening. The DT in the SL isometric twitch was always greater than that in the ML isometric twitch over the range of lengths studied. The slope of the SL-DT relation curve was much less steep than that of the ascending limb of the ML-DT relation curve. The gradient of the SL-DT relationship curve was greater for the long marker than for the short marker distance. The percentage shortening of SL in the ML isometric twitch increased with decreasing ML; it was greater for the short marker than for the long marker distance, particularly at shorter MLs.(ABSTRACT TRUNCATED AT 250 WORDS)