Spatial Nonuniformity of Contraction Causes Arrhythmogenic Ca2+Waves in Rat Cardiac Muscle

Mechanosensitivity of microdomain calcium signalling in the heart

In cardiac myocytes, calcium (Ca²⁺) signalling is tightly controlled in dedicated microdomains. At the dyad, i.e. the narrow cleft between t-tubules and junctional sarcoplasmic reticulum (SR), many signalling pathways combine to control Ca²⁺-induced Ca²⁺ release during contraction. Local Ca²⁺ gradients also exist in regions where SR and mitochondria are in close contact to regulate energetic demands. Loss of microdomain structures, or dysregulation of local Ca²⁺ fluxes in cardiac disease, is often associated with oxidative stress, contractile dysfunction and arrhythmias. Ca²⁺ signalling at these microdomains is highly mechanosensitive. Recent work has demonstrated that increasing mechanical load triggers rapid local Ca²⁺ releases that are not reflected by changes in global Ca²⁺. Key mechanisms involve rapid mechanotransduction with reactive oxygen species or nitric oxide as primary signalling molecules targeting SR or mitochondria microdomains depending on the nature of the mechanical stimulus. This review summarizes the most recent insights in rapid Ca²⁺ microdomain mechanosensitivity and re-evaluates its (patho)physiological significance in the context of historical data on the macroscopic role of Ca²⁺ in acute force adaptation and mechanically-induced arrhythmias. We distinguish between preload and afterload mediated effects on local Ca²⁺ release, and highlight differences between atrial and ventricular myocytes. Finally, we provide an outlook for further investigation in chronic models of abnormal mechanics (eg post-myocardial infarction, atrial fibrillation), to identify the clinical significance of disturbed Ca²⁺ mechanosensitivity for arrhythmogenesis.

Intracellular dyssynchrony of diastolic cytosolic [Ca²⁺] decay in ventricular cardiomyocytes in cardiac remodeling and human heart failure

RATIONALE

Synchronized release of Ca²⁺ into the cytosol during each cardiac cycle determines cardiomyocyte contraction.

OBJECTIVE

We investigated synchrony of cytosolic [Ca²⁺] decay during diastole and the impact of cardiac remodeling.

METHODS AND RESULTS

Local cytosolic [Ca²⁺] transients (1-µm intervals) were recorded in murine, porcine, and human ventricular single cardiomyocytes. We identified intracellular regions of slow (slowCaR) and fast (fastCaR) [Ca²⁺] decay based on the local time constants of decay (TAUlocal). The SD of TAUlocal as a measure of dyssynchrony was not related to the amplitude or the timing of local Ca²⁺ release. Stimulation of sarcoplasmic reticulum Ca²⁺ ATPase with forskolin or istaroxime accelerated and its inhibition with cyclopiazonic acid slowed TAUlocal significantly more in slowCaR, thus altering the relationship between SD of TAUlocal and global [Ca²⁺] decay (TAUglobal). Na⁺/Ca²⁺ exchanger inhibitor SEA0400 prolonged TAUlocal similarly in slowCaR and fastCaR. FastCaR were associated with increased mitochondrial density and were more sensitive to the mitochondrial Ca²⁺ uniporter blocker Ru360. Variation in TAUlocal was higher in pig and human cardiomyocytes and higher with increased stimulation frequency (2 Hz). TAUlocal correlated with local sarcomere relengthening. In mice with myocardial hypertrophy after transverse aortic constriction, in pigs with chronic myocardial ischemia, and in end-stage human heart failure, variation in TAUlocal was increased and related to cardiomyocyte hypertrophy and increased mitochondrial density.

CONCLUSIONS

In cardiomyocytes, cytosolic [Ca²⁺] decay is regulated locally and related to local sarcomere relengthening. Dyssynchronous intracellular [Ca²⁺] decay in cardiac remodeling and end-stage heart failure suggests a novel mechanism of cellular contractile dysfunction.

Increased myofilament Ca2+-sensitivity and arrhythmia susceptibility

Increased myofilament Ca(2+) sensitivity is a common attribute of many inherited and acquired cardiomyopathies that are associated with cardiac arrhythmias. Accumulating evidence supports the concept that increased myofilament Ca(2+) sensitivity is an independent risk factor for arrhythmias. This review describes and discusses potential underlying molecular and cellular mechanisms how myofilament Ca(2+) sensitivity affects cardiac excitation and leads to the generation of arrhythmias. Emphasized are downstream effects of increased myofilament Ca(2+) sensitivity: altered Ca(2+) buffering/handling, impaired energy metabolism and increased mechanical stretch, and how they may contribute to arrhythmogenesis.

Optical mapping of V(m) and Ca(i)(2+) in a model of arrhythmias induced by local catecholamine application in patterned cell cultures

Catecholamines are known to provoke cardiac arrhythmias, but important aspects such as localization of the arrhythmia source in multicellular tissue and exact ionic mechanisms are not well-known. In this work, a multicellular model of arrhythmias caused by local epinephrine application was developed; V (m) and Ca(i)(2+) changes at the arrhythmia source were measured using fluorescent dyes and high-resolution optical mapping. Cultured strands of neonatal rat myocytes (width approximately 0.4 mm) were produced by patterned growth. Epinephrine (1 micromol/l) was applied over an area of 0.3-0.6 mm via two micropipettes, and strands were stimulated by burst pacing. Local epinephrine application caused triggered arrhythmias with cycle lengths of 202-379 ms and duration of >10 s in 9 out of 16 preparations. Optical V(m) mapping demonstrated that in 78% of cases, the source of arrhythmia was located at the boundary of the locally perfused area. Staining with Ca(i)(2+)-sensitive dye Fluo-4 prevented arrhythmia induction in most cases (85%) likely due to Ca(2+) buffering by the dye. Optical Ca(i)(2+) mapping revealed non-propagated Ca(i)(2+) oscillations at the boundary of the locally perfused area in 45% cases. In conclusion, we developed a new model of catecholamine-dependent arrhythmias allowing mapping of V(m) and Ca(i)(2+) at the arrhythmia source with microscopic resolution. The arrhythmias typically originated from the boundary of the epinephrine-perfused area. The location of the arrhythmia source correlated with localized Ca(i)(2+) oscillations suggesting that arrhythmias were caused by Ca(i)(2+) overload at these locations.

Role of Sarcomere Mechanics and Ca2+ Overload in Ca2+ Waves and Arrhythmias in Rat Cardiac Muscle

Ca(2+) release from the sarcoplasmic reticulum (SR) depends on the sarcoplasmic reticulum (SR) Ca(2+) load and the cytosolic Ca(2+) level. Arrhythmogenic Ca(2+) waves underlying triggered propagated contractions arise from Ca(2+) overloaded regions near damaged areas in the cardiac muscle. Ca(2+) waves can also be induced in undamaged muscle, in regions with nonuniform excitation-contraction (EC) coupling by the cycle of stretch and release in the border zone between the damaged and intact regions. We hypothesize that rapid shortening of sarcomeres in the border zone during relaxation causes Ca(2+) release from troponin C (TnC) on thin filaments and initiates Ca(2+) waves. Elimination of this shortening will inhibit the initiation of Ca(2+) waves, while SR Ca(2+) overload will enhance the waves. Force, sarcomere length (SL), and [Ca(2+)](i) were measured and muscle length was controlled. A small jet of Hepes solution with an extracellular [Ca(2+)] 10 mM (HC), or HC containing BDM, was used to weaken a 300 mum long muscle segment. Trains of electrical stimuli were used to induce Ca(2+) waves. The effects of small exponential stretches on triggered propagatory contraction (TPC) amplitude and propagation velocity of Ca(2+) waves (V(prop)) were studied. Sarcomere shortening was uniform prior to activation. HC induced spontaneous diastolic sarcomere contractions in the jet region and attenuated twitch sarcomere shortening; HC+ butanedione monoxime (BDM) caused stretch only in the jet region. Stimulus trains induced Ca(2+) waves, which started inside the HC jet region during twitch relaxation. Ca(2+) waves started in the border zone of the BDM jet. The initial local [Ca(2+)](i) rise of the waves by HC was twice that by BDM. The waves propagated at a V(prop) of 2.0 +/- 0.2 mm/sec. Arrhythmias occurred frequently in trabeculae following exposure to the HC jet. Stretch early during relaxation, which reduced sarcomere shortening in the weakened regions, substantially decreased force of the TPC (F(TPC)) and delayed Ca(2+) waves, and reduced V(prop) commensurate with the reduction F(TPC). These results are consistent with the hypothesis that Ca(2+) release from the myofilaments initiates arrhythmogenic propagating Ca(2+) release. Prevention of sarcomere shortening, by itself, did not inhibit Ca(2+) wave generation. SR Ca(2+) overload potentiated initiation and propagation of Ca(2+) waves.