Titin-based modulation of active tension and interfilament lattice spacing in skinned rat cardiac muscle

The effect of titin-based passive tension on Ca²⁺ sensitivity of active tension and interfilament lattice spacing was studied in skinned rat ventricular trabeculae by measuring the sarcomere length (SL)-dependent change in Ca²⁺ sensitivity and performing small angle X-ray diffraction studies. To vary passive tension, preparations were treated with trypsin at a low concentration (0.31 μg/ml) for a short period (13 min) at 20°C, that resulted in ∼40% degradation of the I-band region of titin, with a minimal effect on A-band titin. We found that the effect of trypsin on titin-based passive tension was significantly more pronounced immediately after stretch than at steady state, 30 min after stretch (i.e., trypsin has a greater effect on viscosity than on elasticity of passive cardiac muscle). Ca²⁺ sensitivity was decreased by trypsin treatment at SL 2.25 μm, but not at SL 1.9 μm, resulting in marked attenuation of the SL-dependent increase in Ca²⁺ sensitivity. The SL-dependent change in Ca²⁺ sensitivity was significantly correlated with titin-based passive tension. Small-angle X-ray diffraction experiments revealed that the lattice spacing expands after trypsin treatment, especially at SL 2.25 μm, providing an inverse linear relationship between the lattice spacing and Ca²⁺ sensitivity. These results support the view that titin-based passive tension promotes actomyosin interaction and that the mechanism includes interfilament lattice spacing modulation.