Does Calcium Sensitivity Increase after Active Stretch in Skinned Muscle Fibres?

Abstract

Introduction Isometric force after active stretch of muscles is higher than the purely isometric force at the corresponding lengths. This property is termed residual force enhancement (RFE). Active force in skeletal muscle depends on calcium attachment characteristics to troponin. Passive force has been shown to influence calcium attachment characteristics in cardiac muscle, specifically the sarcomere length-dependence of Ca-sensitivity. Since one of the mechanisms proposed to explain RFE has been the increase in passive force that results from the engagement of titin upon activation and stretch, our aim was to test whether Ca-sensitivity was changed after active stretch and whether this change was related to titin.Methods Tension-pCa curves were established in three groups of fibres for reference and active stretch contractions at a sarcomere length of 3.0μm. Group 1 (n=13) was formed of skinned fibres without any chemical treatment. Fibres in Group 2 (n=12) were treated with trypsin before establishing the tension-pCa curves. In Group 3, fibres (n=9) were osmotically compressed by dextran T-500 after treatment with trypsin.Results and discussion After active stretch, Ca-sensitivity was increased in Group 1 fibres. When titin was eliminated using trypsin, this increased Ca-sensitivity was abolished. This means that titin is probably responsible for the increase in Ca-sensitivity. The mechanism by which titin may modulate Ca-sensitivity is by reducing the myofilament lattice spacing by generating a radial force that compresses the myofilaments. However, when, in the absence of titin, fibres were compressed, they did not show any difference in Ca-sensitivity between reference and active stretch contractions. These results suggest that Ca-sensitivity increases after active stretch compared to isometric contractions and that titin must be present in order for this increase to occur.