[Release-recovery phenomenon in the frog heart muscle during varied divalent Ca concentration and temperature]

Is stiffness increased during ischemia?

Possible sources of increased ventricular stiffness can be more easily appreciated when pressure and volume patterns are considered as a function of time. A discussion on sources of effective or apparent stiffness or stiffness changes includes viscoelastic properties and active behavior at the muscular level. Chamber geometry and coronary vascular pressure and flow are intrinsic ventricular components. Together with the pressure head and crosstalk as extraventricular components, all these properties are integrated to determine intact heart behavior in late relaxation and diastole.

A Model of Cardiac Muscle Dynamics

A mathematical model is presented describing the time- and length-dependent behavior of cardiac muscle. The model describes a wider variety of experimental data than do previously published models. It incorporates a modification of the Hill equation describing the force-velocity relation. Based on the sliding filament theory, the revised equation includes the effects of finite cross-bridge compliance proposed by A. F. Huxley. The essential simplicity of the Hill equation is retained; however, the model successfully predicts force development during both isometric and isotonic contractions, observed deactivation of the contractile element during isotonic shortening, and the apparent dependence of series elastic stiffness on time after stimulation during quick-release and quick-stretch experiments.

Velocity of shortening of unloaded heart muscle and the length-tension relation

A new technique permits removal of the resting force from the muscle during the course of shortening and study of the maximum velocity of shortening with almost zero load at physiological initial muscle lengths. This unloading has been performed as a sudden unloading to zero total load (zero load clamping), or according to the resting length-tension relations, or according to an arbitrary unloading intermediate between both former modes of unloading. Using these various modes of unloading to zero load during shortening, it has been demonstrated that the maximum velocity of shortening V max remains constant between maximum length (l max ) and a length 12.5% shorter than l max . This unique force (zero load)-velocity (V max )-length (up to 12.5% below l max ) relation is independent of die initial muscle length over this range of lengths, independent of the time after the stimulus over the largest portion of the shortening phase, and independent of the sequence of length change and mode of unloading through which it arrived at zero load within this range of lengths.