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Diverse relaxation rates exist among rat cardiomyocytes isolated from a single myocardial region

Prior studies have shown variation in the functional properties of cardiomyocytes isolated from different regions of the left ventricular myocardium. We found that these region‐dependent variations vanish below a tissue volume of ∼7 mm3 in the adult rat myocardium, revealing a fixed level of intrinsic relaxation rate heterogeneity that is independent of tissue volume. Within these microscopically varying cell populations, fast‐relaxing cells were shown to have elevated phosphorylated troponin I compared to slow‐relaxing cells. Relaxation rate was also correlated with cardiomyocyte length, in that slow‐relaxing cells were longer than fast‐relaxing cells. These results show a new relationship between cardiomyocyte morphology and myofilament relaxation, and suggest that functional diversity among individual myocytes at the microscale may contribute to bulk relaxation of the myocardium.