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

The intrinsic cellular mechanisms by which length regulates myocardial contraction, the basis of the Frank-Starling relation, are uncertain. The aim of this work was to test the hypothesis that passive force, possibly via titin, participates in the modulation of Ca2+ sensitivity of cardiac contractile proteins induced by stretch. Titin degradation by a mild trypsin digestion modulated passive force induced by increasing from 1.9 to 2.3 mu m sarcomere length in skinned rat cardiac cells, Force-pCa curves were established at these two sarcomere lengths after Various durations of trypsin application that induced different passive force levels, They allowed us to evaluate myofilament Ca2+ sensitivity by the pCa of half-maximal activation (pCa(50)), 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 contractile machinery. Passive force measured every 2 min decreased exponentially 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 stretch-induced variation in Ca2+ sensitivity. Then, with further decrease in passive force, Delta pCa(50) decreased. At the lowest passive force investigated 20% of initial passive force, Delta pCa(50) decreased by approximately 55%, These effects were not accompanied by a significant modification of either maximal activated force at pCa 4.5 solution or pCa(50) at 1.9 mu m sarcomere length. This indicates that there was no major functional alteration of the contractile machinery during the protocol as also suggested by contractile and regulatory protein electrophoresis on 2.5-12% gradient and 15% SDS-PAGE gels. Thus, besides modulation induced by the reduced lattice spacing during enhanced heart refilling, Ca2+ sensitivity of the cardiac contractile machinery may be controlled at least partially by internal passive load, which is known to be largely attributable to titin. (C) 1999 Academic Press.