Long QT syndromes are inherited or drug-induced cardiac disorders and characterized by prolonged QT and action potential durations (APD). These disorders are potentially life-threatening because of ventricular arrhythmias often resulting in sudden cardiac death. The effects of various compounds on the APD of cardiomyocytes can be investigated in vitro by recording field potentials with micro-electrodes arrays (MEA). It is well known that the APD and the drug effects are frequency-dependent, but the electrical stimulation of cardiomyocytes on MEA generates large stimulation artifacts and is technically very challenging.

We have therefore developed a novel optogenetic approach for contact-free excitation of cardiomyocytes on MEA. HL-1 cardiomyocytes were genetically engineered to express the light-gated cation channel Channelrhodopsin 2 (ChR2) under the control of the Ef1α promoter and FACS sorting was used to obtain a stable cell line with 92% cells expressing ChR2. These cells were plated on MEA containing 59 electrodes, where they formed a spontaneous beating syncytium. Application of brief light pulsed (10ms, 470nm, 0.39mW/mm²) to the whole MEA enabled artifact-free recording of field-potentials at frequencies from 1.5 to 4Hz. Because global light stimulation led to simultaneous activation of all cardiomyocytes, spatial averaging of field potentials from all electrodes could be performed which improved the signal-to-noise ratio and eliminated inter-electrode variability. We analyzed field potential durations at various beating frequencies and found that these were significantly prolonged (to 126.8±9.1%, n=3) at 4 Hz compared to 2 Hz. Moreover, the potassium channel blocker 4-Aminopyridine significantly increased the field potential duration at all frequencies from 1.5 to 4 Hz (e.g. at 2 Hz to 708±83% or at 4Hz to 550±79%, n=3) in optical paced cardiomyocytes.

Taken together, we present a new method for optical, artifact-free pacing of cardiomyocytes on MEA. This approach enables spatial averaging and determination of frequency-dependent effects of pharmaceutical compounds on field potential durations and hence will be helpful for screening of compounds that treat or induce long QT syndromes.