Titin-isoform dependence of titin-actin interaction and its regulation by S100A1/ Ca²⁺ in skinned myocardium

Titin, also known as connectin, is a large filamentous protein that greatly contributes to passive myocardial stiffness. In vitro evidence suggests that one of titin's spring elements, the PEVK, interacts with actin and that this adds a viscous component to passive stiffness. Differential splicing of titin gives rise to the stiff N2B and more compliant N2BA isoforms. Here we studied the titin-isoformdependence of titin-actin interaction and studied the bovine left atrium (BLA) that expressesmainly N2BA titin, and the bovine left ventricle (BLV) that expresses a mixture of both N2B and N2BA isforms. For comparison we also studiedmouse left ventricular (MLV) myocardium which expresses predominately N2B titin. Using the actin-severing protein gelsolin, we obtained evidence that titin-actin interaction contributes significantly to passive myocardial stiffness in all tissue types, but most in MLV, least in BLA, and an intermediate level in BLV.We also studied whether titin-actin interaction is regulated by S100A1/calcium and found that calcium alone or S100A1 alone did not alter passive stiffness, but that combined they significantly lowered stiffness.We propose that titin-actin interaction is a "viscous break" that is on during diastole and off during systole.