Light diffraction of cardiac muscle: An analysis of sarcomere shortening and muscle tension

Abstract

The time-course of sarcomere shortening and of overall muscle tension were determined for frog atrial strands (30–80 μm diam.) held isometric at a variety of lengths. A light diffraction technique was used to determine the sarcomere length: the cross-section of the strand was illuminated by a continuous He-Ne laser beam. Sarcomeres shortened considerably (up to 0.7 μm), and the velocity of shortening over most of the shortening phase was not affected by stretching the sarcomeres (initial lengths between 1.95 and 3.1 μm) in spite of the increase in muscle tension, (up to 0.65 kg/cm2) and (presumably) a decrease in filament overlap. The incompatibility of these findings with the classical force-velocity relationship of skeletal muscle prompted two critical analyses of this interpretation, first, of the sarcomere length measurements and second, of the assignment of tension to the observed sarcomeres. The conclusions of the first analysis are that the sarcomere length measurements are reliable and that no physiologically important hidden population is likely to be present in the cross-section of the strand. The conclusion of the second analysis is that the assignment of tension to the observed sarcomeres is equivocal since, due to the unavoidably complex morphology of naturally occurring cardiac muscle, there can be no assurity that some unobserved passive element in the cross-section does not bear the active tension. The time-course of sarcomere shortening and of overall muscle tension were determined for frog atrial strands (30–80 μm diam.) held isometric at a variety of lengths. A light diffraction technique was used to determine the sarcomere length: the cross-section of the strand was illuminated by a continuous He-Ne laser beam. Sarcomeres shortened considerably (up to 0.7 μm), and the velocity of shortening over most of the shortening phase was not affected by stretching the sarcomeres (initial lengths between 1.95 and 3.1 μm) in spite of the increase in muscle tension, (up to 0.65 kg/cm2) and (presumably) a decrease in filament overlap. The incompatibility of these findings with the classical force-velocity relationship of skeletal muscle prompted two critical analyses of this interpretation, first, of the sarcomere length measurements and second, of the assignment of tension to the observed sarcomeres. The conclusions of the first analysis are that the sarcomere length measurements are reliable and that no physiologically important hidden population is likely to be present in the cross-section of the strand. The conclusion of the second analysis is that the assignment of tension to the observed sarcomeres is equivocal since, due to the unavoidably complex morphology of naturally occurring cardiac muscle, there can be no assurity that some unobserved passive element in the cross-section does not bear the active tension.