Abstract Body (Do not enter title and authors here): Background. Elucidating key mechanisms of cardiac contraction/relaxation can lead to new treatments for heart failure. Intact papillary muscles produce maximum force at much lower [Ca²⁺] than de-membraned cardiac tissue and show [Ca²⁺] to force hysteresis relationship. We then hypothesize that cross-bridge (CB) attachments sustain force generation independent of [Ca²⁺] after [Ca²⁺] enabling as a key driver of contraction.
Methods. We acquired transgenic mouse model with cardiac myocyte specific endogenous fluorescent calcium sensor GCaMP8. GCaMP8 is green fluorescent protein coupled with calmodulin. We made a novel system using commercial parts and in-house software that can quantify GCaMP8-[Ca²⁺] fluorescence of in vivo beating heart. We then made simultaneous intra-cardiac pressure and [Ca²⁺] measurements of in vivo beating hearts.
Results. By echocardiography, GCaMP8 hearts demonstrated similar size, reduced left ventricular ejection fraction, slowed maximal myocardial contraction velocity S, similar maximal myocardial relaxation velocity e’, and similar E/e’ ratio in comparison to WT. Thus, GCaMP8 hearts have depressed systolic function but preserved diastolic function. Western blotting did not show any expression differences in SERCA2a or phospholamban. Simultaneous intracardiac pressure and [Ca²⁺] measurements showed peak [Ca²⁺] occurring well before peak pressure. Normalizing delay to adjust for different heart rates, peak pressure occurs at 19%±3%SD delay from peak [Ca²⁺] of heart-beat duration, p<0.0001 n=5. Plot of [Ca²⁺] vs. pressure showed hysteresis relationship where pressure rises to peak as [Ca²⁺] falls after initial concordant rise. Pressure rises to only 48%±5%SD of peak at peak [Ca²⁺], p<0.0001 n=5.
Conclusions. We demonstrated CB attachment sustains contraction independent of [Ca²⁺] within beating in vivo hearts. Unlike systole, calcium chelation has little impact on diastolic function. Modulation of CB function can be a new treatment class.