Transient Focal Ischemia in a Mouse Model of Hypertrophic Cardiomyopathy (HCM)
Authors: Shamblin Isaac , Bennuri Sirish , Joensson Daniel , Halade Ganesh , Huke Sabine 
Published: June 2021

IntroductionMice with the troponin T (TnT)‐I79N mutation, which is associated with hypertrophic cardiomyopathy in humans, develop arrhythmias during stress despite the absence of an overt anatomical substrate. We found previously that the arrhythmias are linked to the occurrence of energy deprived regions in the ventricular myocardium, but the underlying mechanism is unknown.ObjectiveThe aims are twofold: 1) to determine if reduced coronary perfusion is responsible for regional energy deprivation and 2) if the regional energy deprivation results in regional contractile dysfunction that can be visualized via high frequency ultrasound.MethodsExperiments were performed in paced isolated Langendorff perfused hearts or in vivo in transgenic and non‐transgenic control mice at 11 to 14 weeks of age. Focal energy deprivation was detected via an increase in Connexin isoforms P0 (Cx43‐P0). Tissue hypoxia was visualized using pimonidazole (hypoxyprobe). Coronary perfusion of Langendorff perfused hearts was assessed via a transonic flow probe. Fluorescent 15 μm microspheres and pimonidazole were infused into isolated hearts to evaluate perfusion into hypoxic regions. To assess heart function non‐invasively in vivo we obtained echocardiographic B‐Mode images (Vevo 3100) which allowed us to take multiple measurements over time in each mouse (2 to 10 months) and analysis included the use of Vevo Strain software.Results

Using pimonidazole, we demonstrate that hypoxia is present in the same regions as focal energy deprivation (correlation coefficient r= 0.76) indicating a local supply‐and‐demand mismatch. However, global coronary flow was not decreased in TnT‐I79N hearts. To test if flow distribution was altered, we infused fluorescent 15 μm microspheres and compared the fraction of microspheres inside hypoxic areas with the fraction of hypoxic tissue. Microspheres inside hypoxic regions were drastically reduced, indicating regional ischemia.
Adding pimonidazole to drinking water, we also demonstrate that hypoxic events occurred spontaneously in vivo. We hypothesized that this could lead to regional contractile deficits that we may be able to detect via ultrasound strain imaging. Fractional shortening and ejection fraction were not altered in TnT‐I79N hearts. However, global longitudinal strain changed greater than 9%, but then recovered in TnT‐I79N mice (2 out of 9). Moreover, when dissected into 2D regions, the regional radial phase velocity deviated below −4.5 % only in TnT‐I79N mice (3 out of 9), which was not observed in control mice.
ConclusionVascular dysfunction and resulting ischemia is at least in part responsible for the regional energy deprivation during stress in TnT‐I79N hearts and may contribute to the transient regional cardiac dysfunction as detected by strain analysis. We can speculate that regional ischemia may contribute to HCM pathology.Support or Funding InformationNIHThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.