Isoproterenol Increases the Fraction of Spark-Dependent RyR-Mediated Leak in Ventricular Myocytes

Recent research suggests that the diastolic ryanodine-receptor-mediated release of Ca²⁺ (J_leak) from the sarcoplasmic reticulum of ventricular myocytes occurs in spark and nonspark forms. Further information about the role(s) of these release manifestations is scarce, however. This study addresses whether the fraction of spark-mediated J_leak increases due to β-adrenergic stimulation. Confocal microscopy was used to simultaneously image Ca²⁺ sparks and quantify J_leak in intact rabbit myocytes, either in the absence or in the presence of 125 nM isoproterenol. It was found that isoproterenol treatment shifts the spark-frequency-J_leak relationship toward an increased sensitivity to a Ca²⁺] trigger. In agreement, a small but significant increase in spark width was found for cells with matched baseline Ca²⁺] and total SR Ca²⁺]. The reconstruction of release fluxes, when applied to the average sparks from those selected cells, yielded a wider release source in the isoproterenol event, indicating the recruitment of peripheral ryanodine receptors. Overall, the results presented here indicate that β-adrenergic stimulation increases the spark-dependent fraction of J_leak. Working together, the increased Ca²⁺ sensitivity and the greater spark width found during isoproterenol treatment may increase the probability of Ca²⁺ wave generation.