Fig. 4: Inhibition of cardiac function to mimic cardiac arrest and bradycardia.

a Fluorescence images showing strong expression of YFP (co-expressed with NpHR) in the heart of a pupal fly. b The hearts of NpHR and WT flies were illuminated constantly for 10 s using red light at the larval, early pupal, and late pupal stages, respectively (see examples in Supplementary video 5–7). The heartbeats of the NpHR flies stopped immediately when red light illumination was applied. The cardiac arrests lasted for 10 s, and regular heartbeats restarted after the light was turned off. M-mode images acquired from WT flies at the three developmental stages are shown as control. Little cardiac function alteration was observed by applying the same constant red-light illumination as for NpHR flies. c Characterization of the inhibition probabilities for different power densities of the stimulation light. d M-mode images showing inhibitory pacing of NpHR flies with three red-light pulse trains at frequencies of 2 Hz, 1 Hz, and 0.6 Hz respectively at larval, early pupal, and late pupal stages (see examples in Supplementary video 8–10). To achieve successful inhibitory pacing, the pulse width was tuned with the given excitation frequency for each pulse train. The heart rates of the fruit flies at the three stages were successfully reduced to 2 Hz, 1 Hz, and 0.6 Hz, even though the RHRs are different. Similar to the cardiac arrest simulation, the heart resumed beating regularly after optical pacing was terminated. Red traces on the M-mode images illustrate the intensity change of excitation light. Red and black fonts in M-mode images represent PRs and HRs, respectively, as in Fig. 2.