Mathematical modeling of the cardiac action potential has proven to be a powerful tool for illuminating various aspects of cardiac function, including cardiac arrhythmias. However, no currently available detailed action potential model accurately reproduces the dynamics of the cardiac action potential and intracellular calcium (Ca(i)) cycling at rapid heart rates relevant to ventricular tachycardia and fibrillation. The aim of this study was to develop such a model. Using an existing rabbit ventricular action potential model, we modified the L-type calcium (Ca) current (I(Ca,L)) and Ca(i) cycling formulations based on new experimental patch-clamp data obtained in isolated rabbit ventricular myocytes, using the perforated patch configuration at 35-37 degrees C. Incorporating a minimal seven-state Markovian model of I(Ca,L) that reproduced Ca- and voltage-dependent kinetics in combination with our previously published dynamic Ca(i) cycling model, the new model replicates experimentally observed action potential duration and Ca(i) transient alternans at rapid heart rates, and accurately reproduces experimental action potential duration restitution curves obtained by either dynamic or S1S2 pacing.

The synchronization of coupled oscillators plays an important role in many biological systems, including the heart. In heart diseases, cardiac myocytes can exhibit abnormal electrical oscillations, such as early afterdepolarizations (EADs), which are associated with lethal arrhythmias. A key unanswered question is how cellular EADs partially synchronize in tissue, as is required for them to propagate. Here, we present evidence, from computational simulations and experiments in isolated myocytes, that irregular EAD behavior is dynamical chaos. We then show in electrically homogeneous tissue models that chaotic EADs synchronize globally when the tissue is smaller than a critical size. However, when the tissue exceeds the critical size, electrotonic coupling can no longer globally synchronize EADs, resulting in regions of partial synchronization that shift in time and space. These regional partially synchronized EADs then form premature ventricular complexes that propagate into recovered tissue without EADs. This process creates multiple hat propagate “shifting” foci resembling polymorphic ventricular tachycardia. Shifting foci encountering shifting repolarization gradients can also develop localized wave breaks leading to reentry and fibrillation. As predicted by the theory, rabbit hearts exposed to oxidative stress (H 2 O 2 ) exhibited multiple shifting foci causing polymorphic tachycardia and fibrillation. This mechanism explains how collective cellular behavior integrates at the tissue scale to generate lethal cardiac arrhythmias over a wide range of heart rates.

Abstract-In the heart, oxidative stress caused by exogenous H 2 O 2 has been shown to induce early afterdepolarizations (EADs) and triggered activity by impairing Na current (I Na ) inactivation. Because H 2 O 2 activates Ca 2ϩ /calmodulin kinase (CaMK)II, which also impairs I Na inactivation and promotes EADs, we hypothesized that CaMKII activation may be an important factor in EADs caused by oxidative stress. Using the patch-clamp and intracellular Ca (Ca i ) imaging in Fluo-4 AM-loaded rabbit ventricular myocytes, we found that exposure to H 2 O 2 (0.2 to 1 mmol/L) for 5 to 15 minutes consistently induced EADs that were suppressed by the I Na blocker tetrodotoxin (10 mol/L), as well as the I Ca,L blocker nifedipine. H 2 O 2 enhanced both peak and late I Ca,L , consistent with CaMKII-mediated facilitation. By prolonging the action potential plateau and increasing Ca influx via I Ca,L , H 2 O 2 -induced EADs were also frequently followed by DADs in response to spontaneous (ie, non-I Ca,L -gated) sarcoplasmic reticulum Ca release after repolarization. The CaMKII inhibitor (1 mol/L; nϭ4), but not its inactive analog (1 mol/L, nϭ5), prevented H 2 O 2 -induced EADs and DADs, and the selective CaMKII peptide inhibitor AIP (autocamtide-2-related inhibitory peptide) (2 mol/L) significantly delayed their onset. In conclusion, H 2 O 2 -induced afterdepolarizations depend on both impaired I Na inactivation to reduce repolarization reserve and enhancement of I Ca,L to reverse repolarization, which are both facilitated by CaMKII activation.

We cloned a cDNA (HAC4) that encodes the hyperpolarization-activated cation channel (I f or I h ) by screening a rabbit sinoatrial (SA) node cDNA library using a fragment of rat brain I f cDNA. HAC4 is composed of 1150 amino acid residues, and its cytoplasmic N-and C-terminal regions are longer than those of HAC1-3. The transmembrane region of HAC4 was most homologous to partially cloned mouse I f BCNG-3 (96%), whereas the C-terminal region of HAC4 showed low homology to all HAC family members so far cloned. Northern blotting revealed that HAC4 mRNA was the most highly expressed in the SA node among the rabbit cardiac tissues examined. The electrophysiological properties of HAC4 were examined using the whole cell patch-clamp technique. In COS-7 cells transfected with HAC4 cDNA, hyperpolarizing voltage steps activated slowly developing inward currents. The half-maximal activation was obtained at ؊87.2 ؎ 2.8 mV under control conditions and at ؊64.4 ؎ 2.6 mV in the presence of intracellular 0.3 mM cAMP. The reversal potential was ؊34.2 ؎ 0.9 mV in 140 mM Na These results indicate that HAC4 forms I f in rabbit heart SA node.