Diastolic Viscoelastic Properties of Rat Cardiac Muscle; Involvement of Ca²⁺

Abstract

Diastolic cardiac sarcomere stiffness, sarcomere length changes, and calcium concentration [Ca²⁺]_i were investigated in 18 trabeculae, dissected from the right ventricle of rat heart. [Ca²⁺]_i declined following a mono-exponential diastolic time course with a time constant of 210–350 ms. During diastole, ([Ca²⁺]_o = 1 mM), sarcomere length (SL) increases (amplitude: 5–65 nm; time constant: 600 ms). Eighty percent of muscles showed discrete spontaneous motion of sarcomeres near the end of diastole; this phenomenon occurred earlier at higher [Ca²⁺]_o. The stiffness modulus of the sarcomere (MOD) increased by 30% during diastole (n=158; p<0.05), while the phase difference, Φ, between force and SL decreased by 13% (n=158; p<0.05). The increase of MOD and the decrease of Φ reversed when spontaneous activation occurred. These results show that the mechanical diastolic properties of the cardiac sarcomere are time dependent. The time dependence of the diastolic properties can be faithfully reproduced by a simple linear four element viscoelastic model. The diastolic changes of MOD and of Φ could be reproduced by assuming an exponential change of the elastic and viscous coefficients of the model over time with a time constant similar to the time constant of change of [Ca²⁺]_i. We suggest that the simplest combination of structural counterparts of the model in the sarcomere consists of titin bound to both actin and myosin in the myofibril, while the sarcomere is in parallel with another purely elastic element. We propose that the Ca²⁺-dependence of diastolic stiffness might be the result of an inverse relation between [Ca²⁺]_i and the affinity of titin for actin.