Osmotic compression and stiffness changes in relaxed skinned cardiac myocytes in PVP-40 and dextran T-500

Sarcomere lengths, cell widths, indices of stiffness, and striation pattern uniformity were determined from radially compressed isolated adult cardiac myocytes from the rat. Single cells were bathed in a series of relaxing solutions containing 0-15% concentrations of nonpenetrating long chain polymers PVP-40 and dextran T-500. There were no significant changes observed in average sarcomere lengths or in striation pattern uniformity at any concentration. But cell widths decreased and stiffness increased in both polymers in a concentration-osmotic pressure-dependent relationship. Changes in cell width and stiffness were repeatable in either polymer, but only after an initial compression with a 10 or 15% concentration solution. The observed reduction in cell width after initial compression correlates well with known myofilament lattice spacing compression in rat cardiac muscle and is qualitatively similar to compressions seen in skeletal muscle preparations. But the cardiac myofilament lattice may not be as compressible as the skeletal lattice. Like skeletal muscle, stiffness exhibits a two-phase relationship where most of the increase occurs at solution osmotic pressures greater than 20 Torr. Finally, the inherently greater passive stiffness-length relationship of cardiac muscle is maintained at higher osmotic pressures such that the passive elastic modulus is strongly length dependent.