Length modulation of active force in rat cardiac myocytes: is titin the sensor?

The intrinsic cellular mechanisms by which length regulates myocardial cont raction, the basis of the Frank-Starling relation, are uncertain. The aim o f this work was to test the hypothesis that passive force, possibly via tit in, participates in the modulation of Ca2+ sensitivity of cardiac contracti le proteins induced by stretch. Titin degradation by a mild trypsin digesti on modulated passive force induced by increasing from 1.9 to 2.3 mu m sarco mere length in skinned rat cardiac cells, Force-pCa curves were established at these two sarcomere lengths after Various durations of trypsin applicat ion that induced different passive force levels, They allowed us to evaluat e myofilament Ca2+ sensitivity by the pCa of half-maximal activation (pCa(5 0)), In control conditions, stretching cells from 1.9 to 2.3 mu m induced a leftward shift of pCa(50) (Delta pCa(50)) of 0.39 +/- 0.03 pCa units (mean +/- SEM, n = 8 cells), reflecting an increase in Ca2+ sensitivity of the c ontractile machinery. Passive force measured every 2 min decreased exponent ially after the beginning of the trypsin application (t(1/2) approximate to 12 min). The first 30% decrease of passive force did not affect the stretc h-induced variation in Ca2+ sensitivity. Then, with further decrease in pas sive force, Delta pCa(50) decreased. At the lowest passive force investigat ed 20% of initial passive force, Delta pCa(50) decreased by approximately 5 5%, These effects were not accompanied by a significant modification of eit her maximal activated force at pCa 4.5 solution or pCa(50) at 1.9 mu m sarc omere length. This indicates that there was no major functional alteration of the contractile machinery during the protocol as also suggested by contr actile and regulatory protein electrophoresis on 2.5-12% gradient and 15% S DS-PAGE gels. Thus, besides modulation induced by the reduced lattice spaci ng during enhanced heart refilling, Ca2+ sensitivity of the cardiac contrac tile machinery may be controlled at least partially by internal passive loa d, which is known to be largely attributable to titin. (C) 1999 Academic Pr ess.