Comparing cardiac action potentials recorded with metal and glass microelectrodes

In Vitro Matured Human Pluripotent Stem Cell-derived Cardiomyocytes Form Grafts With Enhanced Structure and Function in Injured Hearts

BACKGROUND

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) have tremendous promise for application in cardiac regeneration, but their translational potential is limited by an immature phenotype. We hypothesized that large-scale manufacturing of mature hPSC-CMs could be achieved via culture on polydimethylsiloxane (PDMS) lined roller bottles and that the transplantation of these cells would mediate better structural and functional outcomes than with conventional immature hPSC-CM populations.

METHODS

We comprehensively phenotyped hPSC-CMs after in vitro maturation for 20 and 40 days on either PDMS or standard tissue culture plastic (TCP) substrates. All hPSC-CMs were generated using a transgenic hPSC line that stably expressed a voltage-sensitive fluorescent reporter to facilitate in vitro and in vivo electrophysiological studies, and cardiomyocyte populations were also analyzed in vitro by immunocytochemistry, ultrastructure and fluorescent calcium imaging, as well as bulk and single-cell transcriptomics. We next compared outcomes after the transplantation of these populations into a guinea pig model of myocardial infarction (MI) using endpoints including histology, optical mapping of graft- and host-derived action potentials, echocardiography, and telemetric electrocardiographic (ECG) monitoring.

RESULTS

We demonstrated the economic generation of >1x10⁸ mature hPSC-CMs per PDMS-lined roller bottle. Compared to their counterparts generated on TCP substrates, PDMS-matured hPSC-CMs exhibited increased cardiac gene expression and more mature structural and functional properties in vitro. More importantly, intra-cardiac grafts formed with PDMS-matured myocytes showed greatly enhanced structure and alignment, better host-graft electromechanical integration, less pro-arrhythmic behavior, and greater beneficial effects on contractile function.

CONCLUSIONS

In summary, we describe practical methods for the scaled generation of mature hPSC-CMs and provide the first evidence that the transplantation of more mature cardiomyocytes yields better outcomes in vivo.

Optical mapping of human embryonic stem cell-derived cardiomyocyte graft electrical activity in injured hearts

BACKGROUND

Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) show tremendous promise for cardiac regeneration, but the successful development of hESC-CM-based therapies requires improved tools to investigate their electrical behavior in recipient hearts. While optical voltage mapping is a powerful technique for studying myocardial electrical activity ex vivo, we have previously shown that intra-cardiac hESC-CM grafts are not labeled by conventional voltage-sensitive fluorescent dyes. We hypothesized that the water-soluble voltage-sensitive dye di-2-ANEPEQ would label engrafted hESC-CMs and thereby facilitate characterization of graft electrical function and integration.

METHODS

We developed and validated a novel optical voltage mapping strategy based on the simultaneous imaging of the calcium-sensitive fluorescent protein GCaMP3, a graft-autonomous reporter of graft activation, and optical action potentials (oAPs) derived from di-2-ANEPEQ, which labels both graft and host myocardium. Cardiomyocytes from three different GCaMP3+ hESC lines (H7, RUES2, or ESI-17) were transplanted into guinea pig models of subacute and chronic infarction, followed by optical mapping at 2 weeks post-transplantation.

RESULTS

Use of a water-soluble voltage-sensitive dye revealed pro-arrhythmic properties of GCaMP3+ hESC-CM grafts from all three lines including slow conduction velocity, incomplete host-graft coupling, and spatially heterogeneous patterns of activation that varied beat-to-beat. GCaMP3+ hESC-CMs from the RUES2 and ESI-17 lines both showed prolonged oAP durations both in vitro and in vivo. Although hESC-CMs partially remuscularize the injured hearts, histological evaluation revealed immature graft structure and impaired gap junction expression at this early timepoint.

CONCLUSION

Simultaneous imaging of GCaMP3 and di-2-ANEPEQ allowed us to acquire the first unambiguously graft-derived oAPs from hESC-CM-engrafted hearts and yielded critical insights into their arrhythmogenic potential and line-to-line variation.

Detachable glass microelectrodes for recording action potentials in active moving organs

Here, we describe new detachable floating glass micropipette electrode devices that provide targeted action potential recordings in active moving organs without requiring constant mechanical constraint or pharmacological inhibition of tissue motion. The technology is based on the concept of a glass micropipette electrode that is held firmly during cell targeting and intracellular insertion, after which a 100-µg glass microelectrode, a "microdevice," is gently released to remain within the moving organ. The microdevices provide long-term recordings of action potentials, even during millimeter-scale movement of tissue in which the device is embedded. We demonstrate two different glass micropipette electrode holding and detachment designs appropriate for the heart (sharp glass microdevices for cardiac myocytes in rats, guinea pigs, and humans) and the brain (patch glass microdevices for neurons in rats). We explain how microdevices enable measurements of multiple cells within a moving organ that are typically difficult with other technologies. Using sharp microdevices, action potential duration was monitored continuously for 15 min in unconstrained perfused hearts during global ischemia-reperfusion, providing beat-to-beat measurements of changes in action potential duration. Action potentials from neurons in the hippocampus of anesthetized rats were measured with patch microdevices, which provided stable base potentials during long-term recordings. Our results demonstrate that detachable microdevices are an elegant and robust tool to record electrical activity with high temporal resolution and cellular level localization without disturbing the physiological working conditions of the organ.NEW & NOTEWORTHY Cellular action potential measurements within tissue using glass micropipette electrodes usually require tissue immobilization, potentially influencing the physiological relevance of the measurement. Here, we addressed this limitation with novel 100-µg detachable glass microelectrodes that can be precisely positioned to provide long-term measurements of action potential duration during unconstrained tissue movement.

Toward panoramic in situ mapping of action potential propagation in transgenic hearts to investigate initiation and therapeutic control of arrhythmias

To investigate the dynamics and propensity for arrhythmias in intact transgenic hearts comprehensively, optical strategies for panoramic fluorescence imaging of action potential (AP) propagation are essential. In particular, mechanism-oriented molecular studies usually depend on transgenic mouse hearts of only a few millimeters in size. Furthermore, the temporal scales of the mouse heart remain a challenge for panoramic fluorescence imaging with heart rates ranging from 200 min⁻¹ (e.g., depressed sinus node function) to over 1200 min⁻¹ during fast arrhythmias. To meet these challenging demands, we and others developed physiologically relevant mouse models and characterized their hearts with planar AP mapping. Here, we summarize the progress toward panoramic fluorescence imaging and its prospects for the mouse heart. In general, several high-resolution cameras are synchronized and geometrically arranged for panoramic voltage mapping and the surface and blood vessel anatomy documented through image segmentation and heart surface reconstruction. We expect that panoramic voltage imaging will lead to novel insights about molecular arrhythmia mechanisms through quantitative strategies and organ-representative analysis of intact mouse hearts.

Phase Relationship between Alternans of Early and Late Phases of Ventricular Action Potentials

BACKGROUND

Alternans of early phase and of duration of action potential (AP) critically affect dispersion of refractoriness through their influence on conduction and repolarization. We investigated the phase relationship between the two alternans and its effect on conduction.

METHODS AND RESULTS

Transmembrane potentials recorded from ventricles of eight swine and three canines during paced activation intervals of ≤300 ms were used to quantify alternans of maximum rate of depolarization (|dv/dt|(max)) and of action potential duration (APD). Incidence of APD alternans was 62 and 76% in swine and canines. Alternans of APD was frequently accompanied with alternans of |dv/dt|(max). Of these, 4 and 26% were out of phase in swine and canines, i.e., low |dv/dt|(max) preceded long APD. Computer simulations show that out of phase alternans attenuate variation of wavelength and thus minimize formation of spatially discordant alternans.

CONCLUSION

The spontaneous switching of phase relationship between alternans of depolarization and repolarization suggests that mechanisms underlying these alternans may operate independent of each other. The phase between these alternans can critically impact spatial dispersion of refractoriness and thus stability of conduction, with the in phase relation promoting transition from concord to discord while out of phase preventing formation of discord.

Differences in the aging-associated trends of the monophasic action potential duration and effective refractory period of the right and left atria of the rat

BACKGROUND

The incidence of atrial fibrillation (AF) increases with aging, but the aging-associated electrophysiological changes of atrial myocardium are poorly understood.

METHODS AND RESULTS

Based on the hypothesis that aging of the atrium enhances AF susceptibility, 30 Wistar rats were divided into 3 age groups: adult, middle-aged, and aged (n=20 per group). Their hearts were isolated and perfused by Langendorff apparatus. Monophasic action potential duration at 90% repolarization (MAPD(90)) and effective refractory period (ERP) at the basic stimulation cycle length (BCL: 400 ms), and MAPD(90) at other different stimulation cycle lengths in each age group were measured. At the BCL, the MAPD (90) of the right atrial myocardium was prolonged from the adult to the aged group, that of the left atrial myocardium was prolonged from the adult to middle-aged group, and the MAPD(90) of the left atrial myocardium in the aged group were shorter than that in the adult and middle-aged groups. The ERP of the atrial myocardium showed the same age-associated trend as MAPD(90). As the stimulation frequency increased, the MAPD(90) of both the left and right atrial myocardium shortened correspondingly in the adult and middle-aged groups, but in the aged group the MAPD(90) of the right atrial myocardium shortened markedly more than that of the left atrial myocardium.

CONCLUSIONS

There are different aging-associated electrophysiological changes in the right and left atrium, and the older heart is more vulnerable to developing the substrate for AF.

Sildenafil-nitric oxide donor combination promotes ventricular tachyarrhythmias in the swine right ventricle

We tested the hypothesis that sildenafil, singly or in combination with nitric oxide (NO) donors, promotes ventricular tachycardia (VT) and ventricular fibrillation (VF). Vulnerability to VT/VF was tested by rapid pacing in eight isolated normal swine right ventricles (RV). The endocardial activation was optically mapped, and the dynamic action potential duration (APD) restitution curves were constructed with metal microelectrodes. At baseline, no VT/VF could be induced. Sildenafil (0.2 microg/ml) or NO donor singly or in combination did not alter VT/VF vulnerability. However, when 2 microg/ml sildenafil was combined with NO donors, the incidence of VT and VF rose significantly (P < 0.01). VT with a single periodic wavefront was induced in five of eight RVs, and VF with multiple wavefronts was induced in all eight RVs. The sildenafil-NO donor pro-VT/VF combination significantly increased the maximum slope of the APD restitution curve and the amplitude of the APD alternans. The pro-VT/VF effects of sildenafil were reversible after drug-free Tyrode solution perfusion. We conclude that a sildenafil (2 microg/ml) and NO donor combination increases VT/VF vulnerability in the normal RV by a mechanism compatible with the restitution hypothesis.

Effects of amiodarone on wave front dynamics during ventricular fibrillation in isolated swine right ventricle

The effects of acute amiodarone infusion on dynamics of ventricular fibrillation (VF) are unclear. Six isolated swine right ventricles (RVs) were studied in vitro. Activation patterns during VF were mapped optically, whereas action potentials were recorded with a glass microelectrode. At baseline, VF was associated with frequent spontaneous wave breaks. Amiodarone (2.5 microg/ml) reduced spontaneous wave breaks and increased the cycle length (CL) of VF from 83.3 +/- 17.8 ms at baseline to 118.4 +/- 25.8 ms during infusion (P < 0.05). Amiodarone increased the reentrant wave front CL (114.4 +/- 15.5 vs. 78.2 +/- 19.0 ms, P < 0.05) and central core area (4.1 +/- 3.8 vs. 0.9 +/- 0.3 mm2, P < 0.05). Within 30 min of infusion, VF terminated (n = 1), converted to ventricular tachycardia (VT) (n = 1) or continued at a slower rate (n = 4). Amiodarone flattened the APD restitution curves. We conclude that amiodarone reduced spontaneous wave breaks. It might terminate VF or convert VF to VT. These effects were associated with the flattening of APD restitution slope and increased core size of reentrant wave fronts.