Sarcoplasmic Reticulum Ca Release Causes Myocyte Depolarization Underlying Mechanism and Threshold for Triggered Action Potentials

Spontaneous sarcoplasmic reticulum (SR) Ca release causes delayed afterdepolarizations (DADs) via Ca-induced transient inward currents (Iti). However, no quantitative data exists regarding (1) Ca 21 dependence of DADs, (2) Ca required to depolarize the cell to threshold and trigger an action potential (AP), or (3) relative contributions of Ca-activated currents to DADs. To address these points, we evoked SR Ca release by rapid application of caffeine in indo 1-AM–loaded rabbit ventricular myocytes and measured caffeine-induced DADs (cDADs) with whole-cell current clamp. The SR Ca load of the myocyte was varied by different AP frequencies. The cDAD amplitude doubled for every 8868 nmol/L of D[Ca]i (simple exponential), and the D[Ca ]i threshold of 424658 nmol/L was sufficient to trigger an AP. Blocking Na-Ca exchange current (INa/Ca) by removal of [Na]o and [Ca]o (or with 5 mmol/L Ni ) reduced cDADs by .90%, for the same D[Ca]i. In contrast, blockade of Ca-activated Cl current (ICl(Ca)) with 50 mmol/L niflumate did not significantly alter cDADs. We conclude that DADs are almost entirely due to INa/Ca, not ICl(Ca) or Ca -activated nonselective cation current. To trigger an AP requires 30 to 40 mmol/L cytosolic Ca or a [Ca]i transient of 424 nmol/L. Current injection, simulating Itis with different time courses, revealed that faster Itis require less charge for AP triggering. Given that spontaneous SR Ca 21 release occurs in waves, which are slower than cDADs or fast Itis, the true D[Ca ]i threshold for AP activation may be '3-fold higher in normal myocytes. This provides a safety margin against arrhythmia in normal ventricular myocytes. (Circ Res. 2000; 87:774–780.)