Uniform sarcomere shortening behavior in isolated cardiac muscle cells.

Abstract

The dynamics of sarcomere shortening and the diffracting action of single, functionally intact, unattached cardiac muscle cells enzymatically isolated from the ventricular tissue of adult rats were observed. Sarcomere length was measured continuously by a light diffraction method or by direct inspection of the cell''s striated image as recorded on videotape or by cinemicroscopy. At physiological levels of added CaCl2 (0.5-2.0 mM) many cells were quiescent (i.e., they did not beat spontaneously) and contracted in response to electrical stimulation. Sarcomere length in the quiescent, unstimulated cells (1.93 .+-. 0.10 [SD] .mu.m), at peak shortening (1.57 .+-. 0.13 .mu.m, n = 49), and the maximum velocity of sarcomere shortening and relengthening were comparable to previous observations in intact heart muscle preparations. The dispersion of light diffracted by the cell remained narrow, and individual striations remained distinct and laterally well registered throughout the shortening-relengthening cycle. Appreciable nonuniformity and internal buckling were seen at sarcomere lengths < 1.8 .mu.m when the resting cell, embedded in gelatin, was longitudinally compressed. Shortening and relengthening is characterized by uniform activation between myofibrils within the cardiac cell; physiologically significant relengthening forces in living heart muscle originate at the level of the cell rather than in extracellular connections. First-order diffracted light intensity, extremely variable during sarcomere shortening, was always greatest during midrelaxation preceding the onset of a very slow and uniform phase of sarcomere relengthening.