Sarcoplasmic Reticulum Ca 2+ Release Causes Myocyte Depolarization: Underlying Mechanism and Threshold for Triggered Action Potentials

Abstract

Abstract —Spontaneous sarcoplasmic reticulum (SR) Ca 2+ release causes delayed afterdepolarizations (DADs) via Ca 2+ -induced transient inward currents ( I ti ). However, no quantitative data exists regarding (1) Ca 2+ dependence of DADs, (2) Ca 2+ required to depolarize the cell to threshold and trigger an action potential (AP), or (3) relative contributions of Ca 2+ -activated currents to DADs. To address these points, we evoked SR Ca 2+ 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 2+ load of the myocyte was varied by different AP frequencies. The cDAD amplitude doubled for every 88±8 nmol/L of Δ[Ca 2+ ] i (simple exponential), and the Δ[Ca 2+ ] i threshold of 424±58 nmol/L was sufficient to trigger an AP. Blocking Na + -Ca 2+ exchange current ( I Na/Ca ) by removal of [Na] o and [Ca 2+ ] o (or with 5 mmol/L Ni 2+ ) reduced cDADs by >90%, for the same Δ[Ca 2+ ] i . In contrast, blockade of Ca 2+ -activated Cl – current ( I Cl(Ca) ) with 50 μmol/L niflumate did not significantly alter cDADs. We conclude that DADs are almost entirely due to I Na/Ca , not I Cl(Ca) or Ca 2+ -activated nonselective cation current. To trigger an AP requires 30 to 40 μmol/L cytosolic Ca 2+ or a [Ca 2+ ] i transient of 424 nmol/L. Current injection, simulating I ti s with different time courses, revealed that faster I ti s require less charge for AP triggering. Given that spontaneous SR Ca 2+ release occurs in waves, which are slower than cDADs or fast I ti s, the true Δ[Ca 2+ ] i threshold for AP activation may be ≈3-fold higher in normal myocytes. This provides a safety margin against arrhythmia in normal ventricular myocytes.