Fig 7. Representative Ca²⁺ transients from wildtype (WT) and Tnnt2^R141W/+ hearts.

(A) Langendorff perfused hearts were loaded with Rhod-2/AM and imaged on a 16x16 element photodiode array to map Ca²⁺ transients (CaT). Spontaneously beating, Tnnt2^R141W/+ hearts exhibited a markedly slower intrinsic sinus rhythm heart rate (71±12 bpm, top right traces) compared to wildtype hearts (343±52 bpm top left traces; n = 6 each, P<0.03). Calibration of Rhod-2 through measurements of F_max (maximum Rhod-2 fluorescence when saturated with Ca²⁺) and F_min (minimum Rhod-2 fluorescence when all Ca²⁺ is washed out and chelated with EGTA) detected increases in diastolic and peak systolic cytosolic Ca²⁺ in Tnnt2^R141W/+ compared to wildtype hearts (lower left panel). Tnnt2^R141W/+ hearts had a longer rise time of [Ca²⁺]_i, (WT = 14.3±2.09 ms, n = 4 and Tnnt2^R141W/+ = 17.20±0.67, n = 6; P<0.05), and a longer time to recover to diastolic[Ca²⁺]_i. Lower right traces show the superposition of normalized CaTs from a wildtype and a Tnnt2^R141W/+ heart recorded from the center of the left ventricles, when both hearts were paced at the same cycle length (350 ms). The superposition of CaTs exposes marked differences in Ca²⁺ dynamics associated with Ca²⁺ desensitization in Tnnt2^R141W/+ hearts. (B) Tnnt2^R141W/+ hearts exhibited lower intrinsic heart rates and lower peak heart rates in response to isoproterenol at 1 and 10 μM concentrations, compared to wildtype, P<0.001.