Stiffness and shortening changes in myofilament-extracted rat cardiac myocytes

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Abstract
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.