Role of Reduced Sarco-Endoplasmic Reticulum Ca2+-ATPase Function on Sarcoplasmic Reticulum Ca2+ Alternans in the Intact Rabbit Heart

Figures

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  FIGURE 1. Dual optical mapping of Vm (A) and sarcoplasmic reticulum (SR) Ca2+ (B) during sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) inhibition with cyclopiazonic acid (CPA) and pacing at a pacing cycle length (PCL) of 300 ms. CPA caused a dose-dependent increase in APD80 (C) and decrease in normalized SR Ca2+ transient amplitude (D). CPA tended to prolong the time to nadir of SR Ca2+ release (E) and significantly prolonged SR Ca2+ reuptake time (tau, F). Mean ± SD. N = 3–6 (*p < 0.05 vs. baseline).

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  FIGURE 2. Both action potential duration (APD) and sarcoplasmic reticulum (SR) Ca2+ alternans progressively worsen with increased pacing frequency as shown by the alternans magnitude maps (A,B) and in example optical traces (C,D) from the location indicated with a white box in (A,B). In this example, cyclopiazonic acid (CPA) (1–5 μM) caused a dose-dependent increase in the magnitude of SR Ca2+ and APD alternans and an increase in the pacing cycle length (PCL) at which alternans occurred. (E) Example ECG showing T-wave alternans—small (S) vs. large (L) T-wave.

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  FIGURE 3. At pacing cycle length (PCL) = 220 ms, cyclopiazonic acid (CPA) increased the magnitude of both action potential duration (APD) (A) and sarcoplasmic reticulum (SR) Ca2+ (B) alternans. Mean ± SD. N = 3 (*p < 0.05 vs. Baseline). (C,D) Example SR Ca2+ traces at a slower (C) and faster (D) PCL. High-dose CPA (10 μM) tended to decrease alternans magnitude at slower rates, while increasing alternans magnitude at faster rates. (E,F) Quantification of release (E) and load (F) alternans according to the equations indicated. Shaded bars in (C,D) show approximate measurements for calculations in (E,F). S, small; L, large; D, diastolic level. (G) Under all conditions, release alternans emerged at a longer PCL than load alternans. Mean ± SD. N = 3 (*p < 0.05 vs. corresponding threshold for release alternans).

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  FIGURE 4. Sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) inhibition with cyclopiazonic acid (CPA) increased relative ryanodine receptor (RyR) refractoriness (A). (B) S2/S1 ratio (S1 = 300, S2 = 220 ms) was significantly decreased with 1 μM of CPA. Mean ± SD. N = 3 (*p < 0.05 vs. baseline). (C,D) Alternative S1–S2 protocol in which alternans was induced at S1 = 220 ms and a longer S2 (500 ms) was applied. The pacing protocol was repeated so that the last S1 was either a small (solid) or large (dashed) release. A similarly large sarcoplasmic reticulum (SR) Ca2+ release occurred after a large or small release at baseline (C) and following 1 μM of CPA (D), indicating similar SR Ca2+ load after small and large beats.

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  FIGURE 5. Inhibition of sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) with cyclopiazonic acid (CPA) promoted the emergence of spatially discordant alternans at slower pacing cycle lengths (PCL). (A) Magnitude of sarcoplasmic reticulum (SR) Ca2+ alternans at progressively faster PCLs. Spatial discordance emerges at 220-ms PCL for 10 μM of CPA and at 200-ms PCL for 5 μM CPA, while baseline and 1 μM of CPA remain spatially concordant over the PCLs shown. (B) Alternans phase with the nodal line indicated in white where alternans transitions from small:large to large:small. Optical Vm and SR Ca2+ traces are from the regions indicated with white and black boxes. (C) Pacing threshold for the induction of spatial discordance was significantly increased with CPA. Mean ± SD. N = 3 (*p < 0.05 vs. baseline).