Increased calcium spark frequency and variability of action potential duration precede early after depolarisations in isolated ventricular myocytes

Drug-induced inhibition of the delayed rectifier potassium ( I _Kr ) current predisposes to early afterdepolarisations (EADs) and cardiac arrhythmias. Here, we sought to determine the contribution of action potential duration (APD), APD variability and spontaneous calcium release from the sarcoplasmic reticulum (SR) in the formation of EADs. In isolated sheep ventricular myocytes, EADs were induced by combined inhibition of I _Kr with dofetilide and β-adrenergic stimulation. The onset of EADs was preceded by increased beat-to-beat variability of APD. To isolate the role of APD in EAD initiation, the sarcoplasmic reticulum (SR) was depleted of calcium with caffeine. The first beat post-caffeine was associated with prolonged APD but not an EAD. During β-AR stimulation, increasing ryanodine receptor open probability had no effect on APD but increased APD variability and induced both EADs and delayed afterdepolarisations (DADs). Targeting RyR open probability with K201 reversibly abolished afterdepolarisations. APD variability was a better predictor of EADs than APD alone. During an EAD, changes in [Ca ²⁺ ] _i preceded those of membrane depolarisation and the changes in [Ca ²⁺ ] _i were in the form of calcium sparks. In silico modelling demonstrated that membrane time constant effects account for the delay between changes in [Ca ²⁺ ] _i and membrane potential. In summary, using a drug-induced model of action potential prolongation with β-AR stimulation, EADs are preceded by increased APD variability and an increase in Ca ²⁺ sparks. Targeting SR function abolishes EADs. These results suggest a key role for SR Ca ²⁺ overload in the formation of EADs and indicate that EADs and DADs share common mechanisms.