A cross-bridge model for inotropism as revealed by stiffness measurements in cardiac muscle.

Stiffness measurements obtained by means of rapid length changes performed according to Huxley and Simmons (23) showed that the series elasticity of the living frog myocardium obeys Hooke's law and alters in proportion to isometric tension. The same results had previously been reported for glycerol-extracted heart muscle (15, 16). Under conditions of postive inotropism caused by application of noradrenaline, adrenaline or increased extracellular Ca++ concentration, the proportionality between tension and stiffness is maintained (13). As there is strong evidence that the series elasticity of heart muscle resides in the cross-bridges (17, 24) this means that systolic force development and positive inotropism are due to the same process, namely a recruitment of "activated" cross-bridges (an increase in the number of cross-bridges attached to actin at any moment). This rules out the two-component model proposed by Sonnenblick in which a non-linear series elastic element was postulated.