Length-dependent Ca2+ activation in cardiac muscle:: some remaining questions

The steep relationship between systolic force and end diastolic volume in cardiac muscle (Frank-Starling relation) is, to a large extent, based on length-dependent changes in myofilament Ca2+ sensitivity. How sarcomere length modulates Ca2+ sensitivity is still a topic of active investigation. Two general themes have emerged in recent years. On the one hand, there is a large body of evidence indicating that length-dependent changes in lattice spacing determine changes in Ca2+ sensitivity for a given set of conditions. A model has been put forward in which the number of strong-binding cross-bridges that are formed is directly related to the proximity of the myosin heads to binding sites on actin. On the other hand, there is also a body of evidence suggesting that lattice spacing and Ca2+ sensitivity are not tightly linked and that there is a length-sensing element in the sarcomere, which can modulate actin-myosin interactions independent of changes in lattice spacing. In this review, we examine the evidence that has been cited in support of these viewpoints. Much recent progress has been based on the combination of mechanical measurements with X-ray diffraction analysis of lattice spacing and cross-bridge interaction with actin. Compelling evidence indicates that the relationship between sarcomere length and lattice spacing is influenced by the elastic properties of titin and that changes in lattice spacing directly modulate cross-bridge interactions with thin filaments. However, there is also evidence that the precise relationship between Ca2+ sensitivity and lattice spacing can be altered by changes in protein isoform expression, protein phosphorylation, modifiers of cross-bridge kinetics, and changes in titin compliance. Hence although there is no unique relationship between Ca2+ sensitivity and lattice spacing the evidence strongly suggests that under any given set of physiological circumstances variation in lattice spacing is the major determinant of length-dependent changes in Ca2+ sensitivity.