Increased Vulnerability to Atrial Fibrillation Is Associated With Increased Susceptibility to Alternans in Old Sheep

Abstract

Background Atrial fibrillation ( AF ) is common in the elderly, but rare in the young; however, the changes that occur with age that promote AF are not fully understood. Action potential ( AP ) alternans may be involved in the initiation of AF . Using a translationally relevant model, we investigated whether age-associated atrial vulnerability to AF was associated with susceptibility to AP alternans. Methods and Results AF was induced in conscious young and old sheep using 50 Hz burst pacing. Old sheep were more vulnerable to AF . Monophasic and cellular AP s were recorded from the right atrium in vivo and from myocytes isolated from the left and right atrial appendages. AP alternans occurred at lower stimulation frequencies in old sheep than young in vivo (old, 3.0±0.1 Hz; young, 3.3±0.1 Hz; P<0.05) and in isolated myocytes (old, 1.6±0.1 Hz; young, 2.0±0.1 Hz; P<0.05). Simultaneous recordings of [Ca²⁺]_i and membrane potential in myocytes showed that alternans of AP s and [Ca²⁺]_i often occurred together. However, at low stimulation rates [Ca²⁺]_i alternans could occur without AP alternans, whereas at high stimulation rates AP alternans could still be observed despite disabling Ca²⁺ cycling using thapsigargin. Conclusions We have shown, for the first time in a large mammalian model, that aging is associated with increased duration of AF and susceptibility to AP alternans. We suggest that instabilities in Ca²⁺ handling initiate alternans at low stimulation rates, but that AP restitution alone can sustain alternans at higher rates.

Keywords: action potential; aging; alternans; atrial fibrillation; calcium regulation.

Figure 1

Aged atria are more susceptible to atrial fibrillation. A, Surface ECG and right atrial endocardial electrogram before, during, and after 50 Hz stimulation. B, Histogram of maximum AF duration. AF indicates atrial fibrillation; EGM, endocardial electrogram.

Figure 2

Atria from old animals are more susceptible to action potential alternans. A, Monophasic APs stimulated at 3.4, 3.6, and 4 Hz (left) with averaged odd and even traces (right). B, Kaplan‐Meier curves of atria having shown alternans. C, Representative traces from (i) young and (ii) old sheep. D, Relationship between stimulation frequency and alternans magnitude. E, APD ₉₀ of monophasic action potentials stimulated at 2 Hz. *P<0.05. NS indicates not significant; V_{A lt}, spectral magnitude at 0.5 cycles·beat⁻¹; APD ₉₀, action potential duration at 90% repolarization. Differences assessed using log‐rank test and repeated‐measure ANOVA.

Figure 3

No correlation was noted between duration of AF and threshold for (A) whole trace alternans or (B) amplitude alternans. AF indicates atrial fibrillation; NS, not significant.

Figure 4

In vivo (A) and cellular (B) AP restitution curves do not differ between young and old sheep. (i) Examples of AP shortening in response to decreasing DI. (ii) Representative restitution curve with fitted single exponential. (iii) Maximum slope of AP restitution curves. AP indicates action potential; APD ₉₀, action potential duration at 90% repolarization; DI, diastolic interval; D.U., dimensionless units; NS, not significant.

Figure 5

Alternans threshold is lower and action potential duration is prolonged in atrial myocytes from old sheep. A, Kaplan–Meier curve of cells having shown alternans. B, Representative traces showing mean of odd and even sweeps from (i) young and (ii) old atrial myocytes. C, Alternans magnitude from 1 to 4 Hz. D, Mean action potentials±SEMs from left atrial myocytes from young and old sheep. E, APD ₉₀ stimulated from 0.25 to 4 Hz. F, APD ₉₀ from myocytes stimulated at 0.5 Hz stratified by age and laterality. *P<0.05 for difference between age groups in both atria; P<0.05 for difference between left and right atria in both age groups. APD ₉₀ indicates action potential duration at 90% repolarization. Differences assessed using log‐rank test and linear mixed modeling.

Figure 6

A, No correlation was noted between cellular alternans threshold and action potential duration. B, Alternans magnitude was positively correlated with action potential duration. APD ₉₀ indicates action potential duration at 90% repolarization; NS, not significant.

Figure 7

Alternans can affect the repolarization and/or amplitude of action potentials. Monophasic action potentials with: A, Repolarization alternans. B, Amplitude and repolarization alternans. C, Proportion of in vivo traces showing amplitude alternans at the slowest and fastest stimulation rates generating alternans. Transmembrane action potentials with (D) repolarization alternans, (E) (i) amplitude and repolarization alternans (ii) 2:1 refractoriness. F, Proportion of cellular traces showing amplitude alternans at the slowest and fastest stimulation rates generating alternans. *P<0.05.

Figure 8

The relationship between action potential and [Ca²⁺]_i alternans. All recordings made sequentially from the same atrial myocyte. A, [Ca²⁺]_i alternans without AP alternans. B, [Ca²⁺]_i alternans with AP repolarization alternans alone. C, [Ca²⁺]_i alternans with AP amplitude alternans. D, After application of thapsigargin, no alternans of AP or [Ca²⁺]_i at low stimulation rates. E, After thapsigargin, at high stimulation rates, AP alternans still occurs without [Ca²⁺]_i alternans. AP indicates action potential.