Cross-talk, cross-bridges, and calcium activation of cardiac contraction

X-ROS Signaling Depends on Length-Dependent Calcium Buffering by Troponin

The stretching of a cardiomyocyte leads to the increased production of reactive oxygen species that increases ryanodine receptor open probability through a process termed X-ROS signaling. The stretching of the myocyte also increases the calcium affinity of myofilament Troponin C, which increases its calcium buffering capacity. Here, an integrative experimental and modeling study is pursued to explain the interplay of length-dependent changes in calcium buffering by troponin and stretch-activated X-ROS calcium signaling. Using this combination, we show that the troponin C-dependent increase in myoplasmic calcium buffering during myocyte stretching largely offsets the X-ROS-dependent increase in calcium release from the sarcoplasmic reticulum. The combination of modeling and experiment are further informed by the elimination of length-dependent changes to troponin C calcium binding in the presence of blebbistatin. Here, the model suggests that it is the X-ROS signaling-dependent Ca²⁺ release increase that serves to maintain free myoplasmic calcium concentrations during a change in myocyte length. Together, our experimental and modeling approaches have further defined the relative contributions of X-ROS signaling and the length-dependent calcium buffering by troponin in shaping the myoplasmic calcium transient.

Functional significance of C-terminal mobile domain of cardiac troponin I

Ca²⁺-regulation of cardiac contractility is mediated through the troponin complex, which comprises three subunits: cTnC, cTnI, and cTnT. As intracellular [Ca²⁺] increases, cTnI reduces its binding interactions with actin to primarily interact with cTnC, thereby enabling contraction. A portion of this regulatory switching involves the mobile domain of cTnI (cTnI-MD), the role of which in muscle contractility is still elusive. To study the functional significance of cTnI-MD, we engineered two cTnI constructs in which the MD was truncated to various extents: cTnI(1-167) and cTnI(1-193). These truncations were exchanged for endogenous cTnI in skinned rat papillary muscle fibers, and their influence on Ca²⁺-activated contraction and cross-bridge cycling kinetics was assessed at short (1.9 μm) and long (2.2 μm) sarcomere lengths (SLs). Our results show that the cTnI(1-167) truncation diminished the SL-induced increase in Ca²⁺-sensitivity of contraction, but not the SL-dependent increase in maximal tension, suggesting an uncoupling between the thin and thick filament contributions to length dependent activation. Compared to cTnI(WT), both truncations displayed greater Ca²⁺-sensitivity and faster cross-bridge attachment rates at both SLs. Furthermore, cTnI(1-167) slowed MgADP release rate and enhanced cross-bridge binding. Our findings imply that cTnI-MD truncations affect the blocked-to closed-state transition(s) and destabilize the closed-state position of tropomyosin.