Ca²⁺ increases cardiac muscle viscoelasticity independent of active force development

In addition to activation of muscle contraction by Ca²⁺, previous studies suggest that Ca²⁺ also affects muscle passive mechanical properties. The goal of this study was to determine if Ca²⁺ regulates the stiffness of cardiac muscle, independent of active contraction. The mechanical response to stretch for mouse demembranated cardiac trabeculae was probed at different Ca²⁺ levels after eliminating active contraction using a combination of two myosin ATPase inhibitors: para-nitroblebbistatin (PNB; 50 μM) plus mavacamten (Mava; 50 μM). Myocardial force level was assessed during large stretches (≈20% initial muscle length) with a range of stretch velocities. For relaxed muscle, in response to stretch, muscle force rose to a peak and then decayed toward a lower steady-state level. Peak force was higher with faster stretch velocity, consistent with the presence of a viscoelastic element. However, the steady-state force was independent of stretch velocity, consistent with the presence of an elastic component. In the presence of the inhibitors PNB plus Mava, when the Ca²⁺ level was increased, active contraction was completely prevented. However, the viscoelastic force response to stretch was markedly increased by high Ca²⁺ and was >sixfold higher than at the low Ca²⁺ level. The relationship of viscoelastic force to Ca²⁺ level had a similar form to the relationship of active force to Ca²⁺ (measured in the absence of inhibitors), suggesting that a common regulatory mechanism is involved. As expected, Ca²⁺-activated contraction was inhibited by lowering the temperature from 21°C to 10°C. In contrast, the Ca²⁺-activated viscoelastic property was not inhibited at lower temperatures, further suggesting that active contraction and the viscoelastic property involve distinct mechanisms. This study demonstrates that in addition to triggering activation of contraction, Ca²⁺ also increases the apparent viscoelastic property of cardiac muscle.