Diminished Ca ²⁺ Sensitivity of Skinned Cardiac Muscle Contractility Coincident With Troponin T–Band Shifts in the Diabetic Rat

Abstract

We have measured the apparent Ca2+ sensitivities of force development in skinned cardiac trabeculae at different sarcome lengths together with shifts in troponin (Tn) T subunits on specimens from the same hearts and drawn insights into the pathogenesis of myocardial dysfunction in the diabetic rat. The Ca(2+)-force relations were measured at a long (2.4-microns) and a short (1.9-microns) sarcomere length. In disease, compared with the control condition, the apparent Ca2+ sensitivity was greatly diminished at a sarcomere length of 1.9 microns but not affected at all at the long length (2.4 microns). We also examined the alterations in contractile regulatory proteins TnT and TnI by both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blots. The TnI band was largely unperturbed, but major changes were discerned in TnT. The normal rat heart indicated two major bands (TnT1 and TnT2) and a faint third band (TnT3); in the diabetic rat heart, there was a significant shift in intensity from TnT1 to TnT3. Since myosin isozyme shifts also accompany diabetes in the rat, we used a prototypical hypothyroid rat as well to evaluate the myosin influence in the length-induced effects on Ca2+ sensitivity. Myosin shifts during hypothyroidism were unaccompanied by significant changes in TnT, and there were also no length-dependent modifications in Ca2+ sensitivity. The findings raise the possibility that diabetic Ca(2+)-sensitivity changes in the myocardium are coupled with TnT alterations. A plausible explanation is offered whereby these TnT alterations modify the length dependence of Ca2+ sensitivity.

Role of myosin heavy chain composition in kinetics of force development and relaxation in rat myocardium

“…MLC1v and MLC2v) or thin filament proteins (Fig. 1A), confirming previously published findings (Averyhart-Fullard et al 1994;Schiaffino & Reggiani, 1996;Akella et al 1997). Since the cardiac myofibrillar effects of thyroid deficiency are limited to changes in the expression of MHC isoforms, any alterations in contractile properties in skinned myocardium may be attributed to altered MHC expression.…”

“…It is interesting to note that while cardiac MHC expression varies with thyroid state, altered levels of triiodothyronine (T×) do not appear to alter the phenotypic expression of other myofibrillar protein isoforms, such as the thin filament proteins troponin I (TnI) and troponin T (TnT) (Averyhart-Fullard et al 1994;Akella et al 1997). However, it is well known that thyroid state markedly affects the Ca¥ handling properties of the sarcoplasmic reticulum (SR) (Kiss et al 1994).…”

Myofibrillar remodelling in cardiac hypertrophy, heart failure and cardiomyopathies

“…Isoform appearance is developmentally regulated: cTnT 3 predominates, while cTnT 4 is a minor (fetal) isoform in the adult human heart; the additional two fetal isoforms, cTnT 1 and cTnT 2 , are also present, but are barely detectable (86,87). Different studies have suggested that a significant amount of cTnT 4 is re-expressed in failing myocardium (84,85,87), which exhibits decreased myofibrillar ATPase activity (80,81,83) and a decrease in Ca 2+ sensitivity (88,89). Changes in the variable region of the cTnT molecule can alter interactions in the troponin complex; this may explain how a shift in cTnT isoforms alters myocardial contractility and Ca 2+ sensitivity of the cardiac contractile proteins (66,86).…”

Alternative splicing, muscle calcium sensitivity, and the modulation of dragonfly flight performance

“…Modulating muscle power output appears to provide the ecologically important ability to operate at different points along a tradeoff between performance and energetic cost. S triated muscles from a variety of taxa show substantial interand intra-specific variation in the sensitivity of myosin crossbridge activation by calcium (1)(2)(3)(4)(5)(6). Within individual animals, calcium sensitivity of muscle activation varies during ontogeny, training, and disease and appears to be one of the primary ways that striated muscles adjust to changes in contractile regimes (7)(8)(9).…”

“…Troponin T (TnT), one of the components of the troponin-tropomyosin complex, varies in isoform composition during development (14)(15)(16)(17), training (18), and human heart failure (19,20). Changes in TnT isoform composition frequently correlate with variation in the calcium sensitivity of myosin cross-bridge activation (1)(2)(3)(4)(5)(6)(7)(8)(9), and experimental manipulations of TnT isoform composition have been shown to affect the calcium sensitivity of actomyosin ATPase (21). Point mutations in human cardiac TnT alter both calcium sensitivity and myosin cross-bridge kinetics (22,23).…”