Substitution of cardiac troponin C into rabbit muscle does not alter the length dependence of Ca2+ sensitivity of tension.

Abstract

SUMMARY1. The isometric length-tension relationship for cardiac muscle is generally steeper than for skeletal muscle in the physiological range of sarcomere lengths. Recent studies suggest that cardiac troponin C (cTnC) may have intrinsic properties that confer greater length-dependent changes in Ca2+ sensitivity of tension than for skeletal troponin C (sTnC). We tested this hypothesis by characterizing tension-pCa (pCa is -log [Ca2+]) relationships in rabbit skinned psoas muscle fibres at mean sarcomere lengths of 2-32 and 1-87 /tm both before and after partial replacement of endogenous sTnC with cTnC.2. In untreated control fibres, the mid-point (pCa50) of the tension-pCa relationship shifted to lower pCa by 0 15 +002 pCa units, i.e. became less sensitive to Ca2+, when sarcomere length was reduced, and the relationship became steeper.3. Partial extraction of endogenous sTnC and reconstitution with cTnC resulted in no change in the length-dependent shift of pCa50 when reconstitution with cTnC was more than 95 % complete; however, when reconstitution was less than 95 % complete, there were significant increases in the length-dependent shift in pCa50.4. An increase in the length-dependent shift of pCa50 was also observed in fibres from which sTnC was partially extracted, but no cTnC was subsequently re-added.5. We conclude that differences in type of TnC alone are not sufficient to explain differences between skeletal and cardiac muscles in the length dependence of Ca2+ sensitivity of tension.

Increasing mammalian cardiomyocyte contractility with residues identified in trout troponin C

“…Numerous studies have demonstrated that the Ca 2ϩ sensitivity of both cardiac and skeletal myocyte preparations can be altered by replacing endogenous cTnC or skeletal TnC with other TnC isoforms or mutants (17,19,25,27,28). However, this is the first known study in which the Ca 2ϩ sensitivity of force generation in cardiac myocytes has been substantively increased by replacing endogenous cTnC with a mutant McTnC isoform.…”

The degree of activation of cardiac muscle depends on muscle length

“…This hypothesis is based on molecular differences between TnC of cardiac and skeletal muscle, which might be associated with the difference in the force-length relationship in both muscles. However, several studies have refuted this hypothesis 23,24 . In one of them, Moss et al 23 characterized the relation between force produced and activation induced by Ca 2+ , at sarcomere lengths of 2.32 µm and 1.87 µm in membrane-free rabbit skeletal muscle fibers.…”

“…However, several studies have refuted this hypothesis 23,24 . In one of them, Moss et al 23 characterized the relation between force produced and activation induced by Ca 2+ , at sarcomere lengths of 2.32 µm and 1.87 µm in membrane-free rabbit skeletal muscle fibers. Measurements were performed prior to and following substitution of TnC by cardiac muscle TnC to determine whether this was the main reason for the differences in force-length relationships of skeletal and cardiac muscle.…”

Cooperative activation in cardiac muscle: impact of sarcomere length

“…This asymmetry in the force-Ca 2ϩ relationship was quantified by regression analysis of the linearized form of the Hill relationship (Eq. 2) (28,33,34). The average Hill parameters that were obtained via this analysis are summarized in Table 2 (parameters n 1 and n 2 ).…”

“…To assess the asymmetry in the force-Ca 2ϩ relationship, linear regression analysis was performed using a modified, linearized form of the Hill equation (28,33,34) Log ͓F fr /͑1 Ϫ F fr ͔͒ ϭ n ⅐ ͕log ͓͑Ca 2ϩ ͔͒ Ϫ log ͑EC 50 ͖͒ (2) in which Ffr represents force development as a fraction of Fmax. The slope of this relationship is governed by the Hill coefficient (n), while the zero crossing represents the EC50 parameter.…”