O. Cazorla et al., Titin-based modulation of calcium sensitivity of active tension in mouse skinned cardiac myocytes, CIRCUL RES, 88(10), 2001, pp. 1028-1035
We studied the effect of titin-based passive force on the length dependence
of activation of cardiac myocytes to explore whether titin may play a role
in the generation of systolic force. Force-pCa relations were measured at
sarcomere lengths (SLs) of 2.0 and 2.3 mum. Passive tension at 2.3 mum SL w
as varied from approximate to1 to approximate to 10 mN/mm(2) by adjusting t
he characteristics of the stretch imposed on the passive cell before activa
tion. Relative to 2.0 mum Si,, the force-pCa curve at 2.3 mum SL and low pa
ssive tension showed a leftward shift (Delta pCa(50) [change in pCa at half
-maximal activation]) of 0.09 +/-0.02 pCa units while at 2.3 mum SL and hig
h passive tension the shift was increased to 0.25 +/-0.03 pCa units. Passiv
e tension also increased Delta pCa(50) at reduced interfilament lattice spa
cing achieved with dextran. We tested whether titin-based passive tension i
nfluences the interfilament lattice spacing by measuring the width of the m
yocyte and by using small-angle x-ray diffraction of mouse left ventricular
wall muscle. Cell width and interfilament lattice spacing varied inversely
with passive tension, in the presence and absence of dextran. The passive
tension effect on length-dependent activation may therefore result from a r
adial titin-based force that modulates the interfilament lattice spacing.