Abstract
Length-dependent activation of calcium-dependent myocardial force generation provides the basis for the Frank-Starling mechanism. To directly compare the effects of mutations associated with hypertrophic cardiomyopathy and dilated cardiomyopathy, the native troponin complex in skinned trabecular fibers of guinea pigs was exchanged with recombinant heterotrimeric, human, cardiac troponin complexes containing different human cardiac troponin T subunits (hcTnT): hypertrophic cardiomyopathy-associated hcTnTR130C, dilated cardiomyopathy-associated hcTnTΔK210 or the wild type hcTnT (hcTnTWT) serving as control. Force-calcium relations of exchanged fibers were explored at short fiber length defined as 110% of slack length (L0) and long fiber length defined as 125% of L0 (1.25 L0). At short fiber length (1.1 L0), calcium sensitivity of force generation expressed by −log [Ca2+] required for half-maximum force generation (pCa50) was highest for the hypertrophic cardiomyopathy-associated mutation R130C (5.657 ± 0.019), intermediate for the wild type control (5.580 ± 0.028) and lowest for the dilated cardiomyopathy-associated mutation ΔK210 (5.325 ± 0.038). Lengthening fibers from 1.1 L0 to 1.25 L0 increased calcium sensitivity in fibers containing hcTnTR130C (delta-pCa50 = +0.030 ± 0.010), did not alter calcium sensitivity in the wild type control (delta-pCa50 = −0.001 ± 0.010), and decreased calcium sensitivity in fibers containing hcTnTΔK210 (delta-pCa50 = −0.034 ± 0.013). Length-dependent activation indicated by the delta-pCa50 was highly significantly (P < 0.001) different between the two mutations. We hypothesize that primary effects of mutations on length-dependent activation contribute to the development of the diverging phenotypes in hypertrophic and dilated cardiomyopathy.
Highlights
The Frank-Starling law describes the intrinsic ability of the heart ventricle to adapt the systolic stroke volume to the previous diastolic filling
The endogenous troponin complex in the left ventricular skinned fibers from guinea pigs was exchanged by exogenous recombinant human cardiac heterotrimeric troponin complex containing the hcTnC and hcTnI wild type subunits and either hcTnTWT, hcTnTR130C or human cardiac troponin T (hcTnT) K210
The exchange efficiency defined by the ratio of hcTnI per total cTnI was 46 ± 2% for the exchange done with heterotrimeric troponin complexes (hcTn) containing the hcTnTWT, 44 ± 1% for the one containing the hcTnTR130C and 48 ± 2% for the one containing the hcTnT K210
Summary
The Frank-Starling law describes the intrinsic ability of the heart ventricle to adapt the systolic stroke volume to the previous diastolic filling. LDA has been attributed to different mechanisms intrinsic to the sarcomere, involving changes in filament lattice spacing (Hanft et al, 2008), stretch-dependent Ca2+ regulation of troponin (Arteaga et al, 2000; Konhilas et al, 2003; Korte et al, 2012; Zhang et al, 2017), ordering of myosin head orientation (Farman et al, 2011), strain-sensing in titin (Millman and Irving, 1988; Ait Mou et al, 2015; Ait-Mou et al, 2016; Linke, 2018), and the communication between these mechanisms (Ait-Mou et al, 2016; Zhang et al, 2017). Previous studies investigating individual mutations in these regulatory proteins associated with HCM and DCM either mostly reported negative or no consequence of mutations on LDA (Li et al, 2013; Sequeira et al, 2013; Mickelson and Chandra, 2017; Reda and Chandra, 2018, 2019)
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