Dynamics of RR interval upon β-adreneroceptor stimulation. A, B Exemplary tachograms of RR intervals at baseline and during isoprenaline challenge (arrows indicate IP administration of isoprenaline). C, D The same two tachograms at enlarged scales, showing RR intervals during 1 minute (8 to 9 minutes after administration of isoprenaline). E, F Two-second ECG traces from the same mice, around time point 8:40 after administration of isoprenaline. G, H Histograms of RR intervals constructed by analysis of 3 minutes ECG recordings before and after isoprenaline administration.