Tension development and sarcomere length in rat cardiac trabeculae. Evidence of length-dependent activation.

We studied the influence of inotropic factors on the shape of the relation between tension and sarcomere length.Tension measurements were performed on thin trabeculae dissected from the right ventricle of the rat heart.Sarcomere length was measured by laser diffraction techniques and controlled by a servomotor system.The relations between tension and sarcomere length were derived from contractions at various extracellular calcium concentrations Ca 2+ o .The time course of tension development was dependent on both sarcomere length and Ca 2+ o .At all Ca 2+ o , the tension attained during contraction was zero at sarcomere lengths of 1.55-1.60pm and maximal at a sarcomere length of 2.35 /im.Neither a summit nor a descending limb was found in the sarcomere length-tension relation.At Ca 2+ o = 0.5 mM, tension increased linearly with sarcomere length, whereas at Ca 2+ o = 2.5 mM, it approached maximal tension exponentially with sarcomere length.The relations between tension and sarcomere length derived from isometric contractions of the muscle and of sarcomeres were identical, and this suggests that shortening of sarcomeres does not contribute significantly to the effect of Ca 2+ o .The relations between tension and sarcomere length obtained at Ca 2+ o = 0.5 mM from contractions 30 seconds after a potentiating burst of stimuli (4 seconds at 4 Hz) were identical to the relation between tension and sarcomere length at Ca 2+ o = 2.5 mm.Our results are consistent with the hypothesis that cardiac muscle length affects contractile performance by its influence on excitation contraction coupling.CircRes 46: 703704705706707708709710711712713714 1980 THE shape of the relation between tension and sarcomere length in cardiac papillary muscle suggests length-dependent activation of the contractile system (Jewell, 1977).However, there are no data on the effect of inotropic interventions on this curve.Only muscle length-tension curves are available, and these give conflicting results (Jewell, 1977;Huntsman and Stewart, 1977;Sonnenblick, 1962;Bodem et al., 1976).It is known that during isometric contractions there is considerable shortening of sarcomeres in the central region of papillary muscle.This happens at the expense of stretch of damaged regions near the clamps holding the ends of the specimen (Krueger and Pollack, 1975;Pollack and Krueger, 1976;Julian et al., 1976;Julian and Sollins, 1975).The damaged regions as well as the normal region may be influenced by inotropic factors.The interpretation of results of isometric contractions of the muscle is therefore ambiguous.The purpose of the present study was to examine the effect of different calcium concentrations on the relations