Abstract
A major concern about using cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CM) for human disease modeling is their immature phenotype which may result in substantial differences in their physiology when compared to native cardiomyocytes. Here we compare intracellular Ca buffering of hiPSC-CM derived from healthy humans to that of acutely-isolated adult rabbit ventricular CM. hiPSC-CM (21 days post cardiac induction) were plated at low density on matrigel-coated dishes and studied 3-5 days after plating. While hiPSC were significantly smaller than rabbit CM (cell capacity 19±2 pF vs 101±12 pF, p<0.01) and had disorganized contractile filaments on electron microscopy, hiPSC-CM nevertheless exhibited robust troponin C expression, the major cytosolic Ca binding site. To measure cytosolic Ca buffering (Trafford method), caffeine (10 mM) was rapidly applied to release Ca from the SR, which then extruded via the NaCa exchanger (NCX), generating an inward NCX current. Integration of NCX current yielded the total amount of Ca released from the SR. Then Δ[Ca]total was plotted as a function of Δ[Ca]free and maximal cytosolic buffering capacity (Bmax) and Kd calculated. Cytosolic Ca buffering parameters were not significantly different between hiPSC-CM (Bmax=100±14μmol/Lcytosol, Kd=0.61±0.09 μM, n=25) and rabbit-CM (Bmax=117±15 μmol/Lcytosol, Kd=0.61±0.11 μM, n=9). EMD57033 (3 μM), which binds to troponin C and increases cytosolic Ca binding in murine CM, also increased cytosolic Ca binding affinity in hiPSC-CM (Kd=0.39±0.05 μM; p<0.05) without affecting Bmax (96±13 μmol/Lcytosol). Importantly, rates of non-NCX mediated Ca removal were not different between hiPSC-CM and adult rabbit CM. These results demonstrate that hiPSC-CM have cytosolic Ca buffering parameters similar to those of acutely-isolated native CM and therefore could be used for modeling cardiac diseases with altered myofilament Ca binding. Supported by AHA-12POST12080080(DK), NIH-RO1-HL71670(BCK).
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