The Importance of Passive Elements in the Contraction of the Heart

Modelling the mechanical properties of cardiac muscle

A model of passive and active cardiac muscle mechanics is presented, suitable for use in continuum mechanics models of the whole heart. The model is based on an extensive review of experimental data from a variety of preparations (intact trabeculae, skinned fibres and myofibrils) and species (mainly rat and ferret) at temperatures from 20 to 27 degrees C. Experimental tests include isometric tension development, isotonic loading, quick-release/restretch, length step and sinusoidal perturbations. We show that all of these experiments can be interpreted with a four state variable model which includes (i) the passive elasticity of myocardial tissue, (ii) the rapid binding of Ca2+ to troponin C and its slower tension-dependent release, (iii) the kinetics of tropomyosin movement and availability of crossbridge binding sites and the length dependence of this process and (iv) the kinetics of crossbridge tension development under perturbations of myofilament length.

Effect of left atrial filling pressure on the activity of specific myosin isozymes in rat heart

One of the fundamental properties of cardiac muscle is the increase in force generated and work performed with a rise in the resting length of the tissue. There are data to indicate that length-dependent responses of electromechanical coupling and calcium binding by troponin are part of the basis for the pressure-volume relation in the heart. In this study, the contribution of changes in the functional properties of the contractile proteins independent of modification in electromechanical coupling has been examined. Isolated working hearts containing either a mixture of myosin heavy chain (MHC) isozymes (alpha[fast] and beta [slow]) or exclusively the fast MHC have been subjected to left atrial filling pressures (LAPs) between 5 and 20 cm H2O. After 40 minutes at a given LAP, the heart was quickly frozen. The relative activities of calcium- and actin-activated ATPase of V1 and V3 myosin, containing alpha- and beta-MHC, were measured in cryostatic sections of the heart by quantitative histochemistry under conditions for which the concentration of calcium would not be limiting. In hearts containing both isozymes of myosin, the relative enzymatic activity of each isozyme of myosin varied with LAP. At low LAP, V1 was primarily responsible for the enzymatic activity, but as LAP increased the relative contribution of V1 decreased and that of V3 increased. The change in the calcium- and actin-activated activities of the enzyme with change in LAP occurred within 5 minutes and was reversible. In spite of the apparent substitution of enzymatic activity of V3 for V1, total myosin ATPase activity did not decline, but instead remained constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of cardiac contractile proteins. Correlations between physiology and biochemistry

Modulation of the functional properties of the contractile proteins of mammalian heart muscle plays a significant role in the response of the heart to beta-adrenergic stimulation. The most well understood modification is a change in the concentration of calcium ions that is required to activate the contractile system. By means of a cAMP-sensitive phosphorylation of the inhibitory subunit of troponin (TNI), the threshold concentration for activation can be increased as much as 5-fold without changing the maximum calcium-activated force. The protein kinase involved in this regulation is located in the sarcolemma. Cholinergic stimulation causes a dephosphorylation of TNI by a cGMP-sensitive phosphatase. The concentration of calcium ions required to activate contraction also decreases as muscle length increases. This response of the contractile proteins does not involve phosphorylation of TNI. Regulation of the maximum calcium-activated force can take place by a cAMP-sensitive reaction involving a different protein kinase that is located inside the cell. This mechanism involves at least two sequential reactions, one a cAMP-controlled phosphorylation of a protein bound to an intracellular membrane to release an active factor, and the second, an interaction between the active factor and the contractile proteins to enhance the capacity for generating force in the presence of calcium. Phosphorylation of the light chain of myosin is produced by a calmodulin-regulated kinase. The light chain of myosin is partially phosphorylated in the intact heart, but beta-adrenergic stimulation of the heart does not increase the decrease of phosphorylation in parallel with the increase in contractility.