Impact of osmotic compression on sarcomere structure and myofilament calcium sensitivity of isolated rat myocardium

Changes in interfilament lattice spacing have been proposed as the mechanism underlying myofilament length-dependent activation. Much of the evidence to support this theory has come from experiments in which high-molecular-weight compounds, such as dextran, were used to osmotically shrink the myofilament lattice. However, whether interfilament spacing directly affects myofilament calcium sensitivity ([EC.sub.50]) has not been established. In this study, skinned isolated rat myocardium was osmotically compressed over a wide range (Dextran T500; 0-6%), and [EC.sub.50] was correlated to both interfilament spacing and [I.sub.1,1]/[I.sub.1,0] intensity ratio. The latter two parameters were determined by X-ray diffraction in a separate group of skinned muscles. Osmotic compression induced a marked reduction in myofilament lattice spacing, concomitant with increases in both [EC.sub.50] and [I.sub.1,1]/[I.sub.1,0] intensity ratio. However, interfilament spacing was not well correlated with [EC.sub.50] ([r.sup.2] = 0.78). A much better and deterministic relationship was observed between [EC.sub.50] and the [I.sub.1,1]/[I.sub.1,0] intensity ratio ([r.sup.2] = 0.99), albeit with a marked discontinuity at low levels of dextran compression; that is, a small amount of external osmotic compression (0.38 kPa, corresponding to 1% Dextran T500) produced a stepwise increase in the [I.sub.1,1]/[I.sub.1,0] ratio concomitant with a stepwise decrease in [EC.sub.50]. These parameters then remained stable over a wide range of further applied osmotic compression (up to 6% dextran). These findings provide support for a 'switch-like' activation mechanism within the cardiac sarcomere that is highly sensitive to changes in external osmotic pressure. skinned muscle; trabeculae; X-ray diffraction; dextran; osmotic pressure; myofilament length-dependent activation; regulation