The effect of sudden stretches on length-tension-and force-velocity relations of mammalian cardiac muscle

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Abstract
In the isotonically beating cat papillary muscle, a sudden augmentation of the preload initiates a visco-elastic strain retardation, in the course of which the preparations show a marked increase in their isotonic mechanogram amplitudes. It is preceded in the first seconds by a short-term decline. These effects correspond well with the phenomena of stress relaxation. When applying these post-stretch phenomena to the length-tension diagram, it is shown that the isometric and isotonic length-tension curves are significantly shifted to greater distances with respect to the resting length-tension curve, involying even a doubling of the myocardial working capacity in certain cases. Stretch-induced alterations inVmax which were obtained from damped quick-releases, initiated during isometric contractions are almost negligible. On the other handVmax obtained from quick-releases which were started immediately after the onset of isotonic contractions are strongly affected by sudden stretches. The stair-like decline of the mechanograms in the initial phase of relaxation or retardation is assumed to be mediated by a stretch-induced alteration in the time course of the action potential. The additional reduction in myocardial performance superimposed on this, is consistent with the hypothesis of a stretch-induced transient inactivation or a delayed activation of contractile interaction sites.