Spontaneous calcium release in tissue from the failing canine heart

Abstract

Abnormalities in calcium handling have been implicated as a significant source of electrical instability in heart failure (HF). While these abnormalities have been investigated extensively in isolated myocytes, how they manifest at the tissue level and trigger arrhythmias is not clear. We hypothesize that in HF, triggered activity (TA) is due to spontaneous calcium release from the sarcoplasmic reticulum that occurs in an aggregate of myocardial cells (an SRC) and that peak SCR amplitude is what determines whether TA will occur. Calcium and voltage optical mapping was performed in ventricular wedge preparations from canines with and without tachycardia-induced HF. In HF, steady-state calcium transients have reduced amplitude [135 vs. 170 ratiometric units (RU), P < 0.05] and increased duration (252 vs. 229 s, P < 0.05) compared with those of normal. Under control conditions and during beta-adrenergic stimulation, TA was more frequent in HF (53% and 93%, respectively) compared with normal (0% and 55%, respectively, P < 0.025). The mechanism of arrhythmias was SCRs, leading to delayed afterdepolarization-mediated triggered beats. Interestingly, the rate of SCR rise was greater for events that triggered a beat (0.41 RU/ms) compared with those that did not (0.18 RU/ms, P < 0.001). In contrast, there was no difference in SCR amplitude between the two groups. In conclusion, TA in HF tissue is associated with abnormal calcium regulation and mediated by the spontaneous release of calcium from the sarcoplasmic reticulum in aggregates of myocardial cells (i.e., an SCR), but importantly, it is the rate of SCR rise rather than amplitude that was associated with TA.

Fig. 1.

Calcium (Ca²⁺) transient amplitude and duration in normal and failing wedge preparations. A: representative Ca²⁺ transients (left) and summary data (right) for ratiometric Ca²⁺ transients amplitude (Amp) from failing (HF, n = 12) and normal (N, n = 9) wedge preparations. Amplitude was significantly reduced in HF relative to normal [135 ± 4 ratiometric units (RU) vs. 170 ± 3 RU, respectively, P < 0.05]. B: Ca²⁺ transients with normalized amplitudes to emphasize differences in duration (left) and summary data (right) for ratiometric Ca²⁺ transients duration at 50% decay (CaD₅₀) from HF and N preparations. HF transients are prolonged with respect to N (252 ± 2 vs. 229 ± 2 ms, respectively, P < 0.05).

Fig. 2.

Triggered activity (TA) and subthreshold SCR events in wedge preparations under control (CNTL) and isoproterenol (Iso) conditions. A: representative ECG, action potentials (see V_m trace), and Ca²⁺ transients recorded during the induction of a triggered beat (TA) by rapid pacing (S1) in a HF preparation during Iso administration. Below the 3 separate traces for ECG, transmembrane potential (V_m), and Ca²⁺ are superimposed traces for V_m and Ca²⁺ on an expanded time scale during the triggered beat. The occurrence of TA was significantly greater in HF-Iso conditions compared with HF-CNTL conditions. B: representative ECG, action potentials, and Ca²⁺ transients recorded during the induction of a subthreshold SCR and delayed afterdepolarization (DAD) in a HF preparation. Below the 3 separate traces for ECG, V_m, and Ca²⁺ are superimposed traces for V_m and Ca²⁺ on an expanded time scale during the SCR/DAD. Summary data show that SCR activity was significantly greater in HF compared with N. The numbers in parentheses are the number of preparations demonstrating the event by the total number of preparations.

Fig. 3.

Rate dependence of SCR amplitude and SCR maximum rate of rise in HF preparations with Iso. A: in HF preparations in which SCRs occurred at multiple cycle lengths (CLs), decreases in CL resulted in increased SCR amplitude and SCR rate of rise measured at the same site on the transmural surface. The percent change in SCR amplitude and SCR rate of rise as a function of decreasing CL were fit with an exponential (solid curves, R² = 0.98 and R² = 0.97, respectively). B: linear regression of SCR rate of rise with SCR amplitude showing a strong, positive correlation (r = 0.92) with R² = 0.84.

Fig. 4.

Left: simultaneously recorded traces for ECG, V_m, and Ca²⁺ during rapid pacing-induced (S1) SCR (arrow) and TA in HF preparations with Iso. Right, top: transmural contour maps (14 × 14 mm) of action potential activation times (in ms) and cytoplasmic calcium levels [in arbitrary units (AU)] during select time points of the last paced beat (third S1, pacing symbol). All times are relative to the onset of electrical activity. Right, bottom: transmural contour maps of action potential activation times (in ms) and cytoplasmic calcium levels (in AU) during select time points of the triggered beat and preceding SCR, respectively. All times are relative to the onset of earliest SCR activity. The V_m activation map of the triggered beat shows that the beat originated focally, precisely at the same location as the SCR. Transmural contour maps are shown with the epicardium (Epi) on the left and the endocardium (Endo) on the right.

Fig. 5.

Shown are traces of 2 SCR events recorded from the same site in a single HF preparation: one that resulted in a triggered beat (A) and one that did not (B). In A, above the last paced beat (S1) are 3 frames showing transmural (14 × 14 mm) calcium level (amplitude) at select time points, demonstrating the rapid, uniform pattern of calcium release during pacing (pacing symbol). Below the trace are several frames of calcium levels from select time points during the SCR and subsequent triggered beat (TA). All times shown are relative to the earliest site of calcium release, during pacing (top) or the SCR (bottom). The Epi and Endo are shown on the left and right sides of each contour, respectively. For the color scale, black corresponds to diastolic calcium, red corresponds to subthreshold SCR, and the transition from red to green corresponds to the threshold for TA in A. Calcium release is much slower during the SCR compared with pacing (see text for details). In B, the exact same format is shown, except that frames of calcium levels during pacing are not shown since they are identical to A. The color scale created for A was also used in B. In A and B, SCRs occur in a relatively large aggregate of myocardial cells and achieved a similar amplitude; however, the rate of SCR rise is much greater in A when TA occurred compared with B when TA did not (see text for details).

Fig. 6.

Relationship between SCR amplitude (A) and SCR rate of rise (B) with the occurrence of triggered beats in HF preparations with Iso. Histograms of SCR amplitude (A) show that SCRs that lead to TA (TA, green) statistically do not have larger amplitudes than those SCR events that fail to trigger a beat (no TA, red). This is indicated by the overlapping histograms. In contrast, histograms of the rate of SCR rise (B) show that SCRs that result in TA (green) have a significantly faster (shifted to the right) rate of rise than SCRs that do not trigger beats (red).