Alterations in the Ca2+ sensitivity of tension development by single skeletal muscle fibers at stretched lengths

Abstract

The apparent length dependence in the calcium sensitivity of tension development in skeletal muscle has been investigated in the present study. At sarcomere lengths of 2.46-2.62 micron, the Hill plot of tension-pCa data is well fit by not one but two straight lines, suggesting the possible involvement of more than a single class of Ca2+-binding site in tension development. On the other hand, increasing the sarcomere length to 3.00-3.25 micron yielded Hill plots that were described by a single straight line, which indicates that at long lengths tension might be regulated by the binding of Ca2+ to a single class of Ca2+-binding sites, presumably the low affinity sites of TnC. This length-dependent transformation of the tension pCa relation occurred at free Mg2+ concentrations of both 0.05 and 3.2 mM. Although the mechanism of this effect is uncertain, plausible explanations for the biphasic Hill plot at the shorter lengths include the possible involvement of Ca2+ activation of the thick filaments and/or myosin LC2 phosphorylation in the process of tension development.

During muscle atrophy, thick, but not thin, filament components are degraded by MuRF1-dependent ubiquitylation

“…Removal of myofibrillar MyLC2 has a similar effect on the Ca 2+ -tension curve as removal of MyBP-C. Both proteins thus appear to reduce inappropriate contractions by decreasing the probability of the myosin heads binding to actin (Moss et al, 1983). In addition, in heart, increasing MyBP-C content enhances the ATPase activity of actomyosin but not if MyLC2 was removed, and this effect reappeared when MyLC2 was restored (Margossian, 1985).…”

Cooperative activation in cardiac muscle: impact of sarcomere length

“…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.…”

“…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 ).…”

Expression and functional properties of four slow skeletal troponin T isoforms in rat muscles

“…where P/P0 is the normalized tension, [Ca 2ϩ ] is Ca 2ϩ concentration, and K is the apparent dissociation constant (pK ϭ Ϫlog K ϭ pCa50); n1 corresponds to P/P0 Ͼ 50% and n2 to P/P0 Ͻ 50% (23).…”