Fig. 5. Baseline atrial and ventricular heart rates of MLC2a-null and wild-type littermate embryos. Mean heart rates at baseline are shown for both cardiac atria and ventricles of wild-type (WT) and MLC2a-null (KO) mouse embryos at ED 9.5. Mean heart rates, measured in beats per minute (bpm), were 51±6 and 4±4 for WT and KO atria, and 53±10 and 48±7 for WT and KO ventricles, respectively.