Stiffness and shortening changes in myofilament-extracted rat cardiac myocytes

Sarcomere lengths, cell widths, volumes, stiffness, and regional striation uniformity were determined from isolated adult cardiac myocytes. Single cells were examined in the control saline solution followed by a sequence of relaxing, membrane skinning, and myofilament extraction solutions. Cell size and shape parameters were determined from freely dispersed myocytes, whereas stiffness was measured from myocytes attached to a perturbator and tension transducer with micropipettes. There were small changes in cell appearance, size, shape, and stiffness in the relaxing and skinning solutions. However, in 0.17-0.56 M KCl myosin extraction media, cell length declined significantly to 1.19 microns, and stiffness fell to 5-10% of control. The rate of cell shortening and stiffness decline was dependent on KCl concentration and pH. Subsequent exposure to higher ionic strength 0.60 M KI thin filament extraction media elicited additional decreases in stiffness (less than 5% of control) and cell length (0.98 micron). Cell shortening and stiffness decline have similar time courses under the same conditions, and they appear to coincide with A-band disassembly as indicated by electron micrographs. These data suggest that cardiac myocyte stiffness, size, and shape are determined in part by a stressed cytoskeleton that is associated with the myofilament apparatus.