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.
KeywordsCalcium sensitivity Myocardium Sarcomere length X-ray diffraction
This work is supported by a National Institutes of Health (NIH) Grant HL62881 (to H.G.). N.F. is a recipient of a grant from the Uehara Memorial Foundation (Tokyo, Japan). We would like to thank J. Henry, G.A. Kumar, and A. Joshi for help with X-ray data analysis. Use of the Advanced Photon Source was supported by the US Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract No. W-31-109-ENG-38. BioCAT is an NIH-supported Research Center (RR08630).
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