Optogenetic quantification of cardiac excitability and electrical coupling in intact hearts to explain cardiac arrhythmia initiation

Increased cardiac excitability and reduced electrical coupling promote cardiac arrhythmia and can be quantified by input resistance (<i>R</i>_m), pacing threshold (<i>I</i>_thr), and cardiac space constant (λ). However, their measurement in the heart was not feasible because the required homogenous current injection cannot be performed with electrical stimulation. We overcame this problem by optogenetic current injection into all illuminated cardiomyocytes of mouse hearts in different action potential phases. Precisely triggered and patterned illumination enabled measuring <i>R</i>_m and λ, which both were smallest at diastole. Pharmacological and depolarization-induced reduction of inwardly rectifying K⁺ currents (<i>I</i>_K1), gap junction block, and cardiac infarction reduced <i>I</i>_thr, showing the importance of high <i>I</i>_K1 density and intact cardiomyocyte coupling for preventing arrhythmia initiation. Combining optogenetic current injection and computer simulations was used to classify pro- and anti-arrhythmic mechanisms based on their effects on <i>R</i>_m and <i>I</i>_thr and allowed to quantify <i>I</i>_K1 inward rectification in the intact heart, identifying reduced <i>I</i>_K1 rectification as anti-arrhythmic concept.