An electric field mechanism for transmission of excitation between myocardial cells

Action potential repolarization enabled by Ca++ channel deactivation in PSpice simulation of smooth muscle propagation

BACKGROUND

Previously, only the rising phase of the action potential (AP) in cardiac muscle and smooth muscle could be simulated due to the instability of PSpice upon insertion of a second black box (BB) into the K+ leg of the basic membrane unit. This restriction was acceptable because only the transmission of excitation from one cell to the next was investigated.

METHODS

In the current work, the repolarization of the AP was accomplished by inserting a second BB into the Ca++ leg of the basic membrane unit. Repolarization of the AP was produced, not through an activation of the K+ channel conductance, but rather through a mimicking of the deactivation of the Ca++ channel conductance. Propagation of complete APs was studied in a chain (strand) of 10 smooth muscle cells, in which various numbers of gap-junction (gj) channels (assumed to be 100 pS each) were inserted across the cell junctions.

RESULTS

The shunt resistance across the junctions produced by the gj-channels (Rgj) was varied from 100,000 MOmega (0 gj-channels) to 10,000 MOmega (1 gj-channel), to 1,000 MOmega (10 channels), to 100 MOmega (100 channels), to 10 MOmega (1000 channels), and to 1.0 MOmega (10,000 channels). Velocity of propagation (theta, in cm/sec) was calculated from the measured total propagation time (TPT, the time difference between when the AP rising phase of the first cell and the last cell crossed -20 mV), assuming a constant cell length of 200 microm. When there were no gj-channels, or only one, the transmission of excitation between cells was produced by the electric field (EF), i.e., the negative junctional cleft potential, that is generated in the narrow junctional clefts (e.g., 100 A) when the prejunctional membrane fires an AP (a fraction of a millisecond before the adjacent surface membrane). There were significant end-effects at the termination of the strand, such that the last cell (cell #10) failed to fire, or fired after a prolonged delay. This end-effect was abolished when the strand termination resistance (Rbt) was increased from 1.0 KOmega to 600 MOmega. When there were 1000 or 10,000 gj-channels, the transmission of excitation was produced by local-circuit current flow from one cell to the next through the gj-channels.

DISCUSSION

In summary, it is now possible to simulate complete APs in smooth muscle cells that could propagate along a single chain of 10 cells, even when there were no gj-channels between the cells.

Repolarization of the action potential enabled by Na+ channel deactivation in PSpice simulation of cardiac muscle propagation

BACKGROUND

In previous studies on propagation of simulated action potentials (APs) in cardiac muscle using PSpice modeling, we reported that a second black-box (BB) could not be inserted into the K+ leg of the basic membrane unit because that caused the PSpice program to become very unstable. Therefore, only the rising phase of the APs could be simulated. This restriction was acceptable since only the mechanism of transmission of excitation from one cell to the next was being investigated.

METHODS AND RESULTS

We have now been able to repolarize the AP by inserting a second BB into the Na+ leg of the basic units. This second BB effectively mimicked deactivation of the Na+ channel conductance. This produced repolarization of the AP, not by activation of K+ conductance, but by deactivation of the Na+ conductance. The propagation of complete APs was studied in a chain (strand) of 10 cardiac muscle cells, in which various numbers of gap-junction (gj) channels (assumed to be 100 pS each) were inserted across the cell junctions. The shunt resistance across the junctions produced by the gj-channels (Rgj) was varied from 100,000 M? (0 gj-channels) to 10,000 M? (1 gj-channel), to 1,000 M? (10 channels), to 100 M? (100 channels), and 10 M? (1000 channels). The velocity of propagation (theta, in cm/s) was calculated from the measured total propagation time (TPT, the time difference between when the AP rising phase of the first cell and the last cell crossed -20 mV, assuming a cell length of 150 microm. When there were no gj-channels, or only a few, the transmission of excitation between cells was produced by the electric field (EF), i.e. the negative junctional cleft potential, that is generated in the narrow junctional clefts (e.g. 100 A) when the prejunctional membrane fires an AP. When there were many gj-channels (e.g. 1000 or 10,000), the transmission of excitation was produced by local-circuit current flow from one cell to the next through the gj-channels.

CONCLUSION

We have now been able to simulate complete APs in cardiac muscle cells that could propagate along a single chain of 10 cells, even when there were no gj-channels between the cells.

Effect of sodium channel blocker, pilsicainide hydrochloride, on net inward current of atrial myocytes in thyroid hormone toxicosis rats

To investigate effect of pilsicainide hydrochloride (pilsicainide) on electrocardiogram (ECG) signals and action potentials (APs) of atrial myocytes, levo-thyroxine (T4, 500 microg/kg body weight) was daily injected into peritoneal cavity of Sprague-Dawley rats for 14 days. T4-treatment significantly shortened RR interval, P wave, and QRS complex durations on ECG. Although pilsicainide did not affect the heart rate, P wave and corrected QT interval (QTc) was increased in T4-treated rats. AP recordings revealed that AP durations at 20%, 50%, and 90% repolarization were significantly shortened and maximal rate of rise (Max dV/dt) was significantly increased in T4-treated rat atrial cells. Pilsicainide significantly decreased AP amplitude (APA) and Max dV/dt in both control and T4-treated rat atrial cells. Concentration-inhibition study demonstrated that pilsicainide significantly inhibited net inward current of T4-treated rats at lower concentration (IC50 of 29.2 microg/mL) than that of control rats (133 microg/mL). In conclusion, pilsicainide could decrease the conduction velocity in T4-treated rat atrium by decreasing the Max dV/dt and net inward current, which could be a possible treatment of thyrotoxicosis-induced arrhythmia.

A Finite Volume Method for Modeling Discontinuous Electrical Activation in Cardiac Tissue

This paper describes a finite volume method for modeling electrical activation in a sample of cardiac tissue using the bidomain equations. Microstructural features to the level of cleavage planes between sheets of myocardial fibers in the tissue are explicitly represented. The key features of this implementation compared to previous modeling are that it represents physical discontinuities without the implicit removal of intracellular volume and it generates linear systems of equations that are computationally efficient to construct and solve. Results obtained using this method highlight how the understanding of discontinuous activation in cardiac tissue can form a basis for better understanding defibrillation processes and experimental recordings.

Propagated repolarization of simulated action potentials in cardiac muscle and smooth muscle

BACKGROUND

Propagation of repolarization is a phenomenon that occurs in cardiac muscle. We wanted to test whether this phenomenon would also occur in our model of simulated action potentials (APs) of cardiac muscle (CM) and smooth muscle (SM) generated with the PSpice program.

METHODS

A linear chain of 5 cells was used, with intracellular stimulation of cell #1 for the antegrade propagation and of cell #5 for the retrograde propagation. The hyperpolarizing stimulus parameters applied for termination of the AP in cell #5 were varied over a wide range in order to generate strength / duration (S/D) curves. Because it was not possible to insert a second "black box" (voltage-controlled current source) into the basic units representing segments of excitable membrane that would allow the cells to respond to small hyperpolarizing voltages, gap-junction (g.j.) channels had to be inserted between the cells, represented by inserting a resistor (Rgj) across the four cell junctions.

RESULTS

Application of sufficient hyperpolarizing current to cell #5 to bring its membrane potential (Vm) to within the range of the sigmoidal curve of the Na+ conductance (CM) or Ca++ conductance (SM) terminated the AP in cell #5 in an all-or-none fashion. If there were no g.j. channels (Rgj = infinity), then only cell #5 repolarized to its stable resting potential (RP; -80 mV for CM and -55 mV for SM). The positive junctional cleft potential (VJC) produced only a small hyperpolarization of cell #4. However, if many g.j. channels were inserted, more hyperpolarizing current was required (for a constant duration) to repolarize cell #5, but repolarization then propagated into cells 4, 3, 2, and 1. When duration of the pulses was varied, a typical S/D curve, characteristic of excitable membranes, was produced. The chronaxie measured from the S/D curve was about 1.0 ms, similar to that obtained for muscle membranes.

CONCLUSIONS

These experiments demonstrate that normal antegrade propagation of excitation can occur in the complete absence of g.j. channels, and therefore no low-resistance pathways between cells, by the electric field (negative VJC) developed in the narrow junctional clefts. Because it was not possible to insert a second black-box into the basic units that would allow the cells to respond to small hyperpolarizing voltages, only cell #5 (the cell injected with hyperpolarizing pulses) repolarized in an all-or-none manner. But addition of many g.j. channels allowed repolarization to propagate in a retrograde direction over all 5 cells.

Gap-junction channels inhibit transverse propagation in cardiac muscle

The effect of adding many gap-junctions (g-j) channels between contiguous cells in a linear chain on transverse propagation between parallel chains was examined in a 5 × 5 model (5 parallel chains of 5 cells each) for cardiac muscle. The action potential upstrokes were simulated using the PSpice program for circuit analysis. Either a single cell was stimulated (cell A1) or the entire chain was stimulated simultaneously (A-chain). Transverse velocity was calculated from the total propagation time (TPT) from when the first AP crossed a V_m of -20 mV and the last AP crossed -20 mV. The number of g-j channels per junction was varied from zero to 100, 1,000 and 10,000 (R_gj of ∞, 100 MΩ, 10 MΩ, 1.0 MΩ, respectively). The longitudinal resistance of the interstitial fluid (ISF) space between the parallel chains (R_ol2) was varied between 200 KΩ (standard value) and 1.0, 5.0, and 10 MΩ. The higher the R_ol2 value, the tighter the packing of the chains. It was found that adding many g-j channels inhibited transverse propagation by blocking activation of all 5 chains, unless R_ol2 was greatly increased above the standard value of 200 KΩ. This was true for either method of stimulation. This was explained by, when there is strong longitudinal coupling between all 5 cells of a chain awaiting excitation, there must be more transfer energy (i.e., more current) to simultaneously excite all 5 cells of a chain.

Functional integration of electrically active cardiac derivatives from genetically engineered human embryonic stem cells with quiescent recipient ventricular cardiomyocytes: insights into the development of cell-based pacemakers

BACKGROUND

Human embryonic stem cells (hESCs) derived from blastocysts can propagate indefinitely in culture while maintaining pluripotency, including the ability to differentiate into cardiomyocytes (CMs); therefore, hESCs may provide an unlimited source of human CMs for cell-based therapies. Although CMs can be derived from hESCs ex vivo, it remains uncertain whether a functional syncytium can be formed between donor and recipient cells after engraftment.

METHODS AND RESULTS

Using a combination of electrophysiological and imaging techniques, here we demonstrate that electrically active, donor CMs derived from hESCs that had been stably genetically engineered by a recombinant lentivirus can functionally integrate with otherwise-quiescent, recipient, ventricular CMs to induce rhythmic electrical and contractile activities in vitro. The integrated syncytium was responsive to the beta-adrenergic agonist isoproterenol as well as to other pharmacological agents such as lidocaine and ZD7288. Similarly, a functional hESC-derived pacemaker could be implanted in the left ventricle in vivo. Detailed optical mapping of the epicardial surface of guinea pig hearts transplanted with hESC-derived CMs confirmed the successful spread of membrane depolarization from the site of injection to the surrounding myocardium.

CONCLUSIONS

We conclude that electrically active, hESC-derived CMs are capable of actively pacing quiescent, recipient, ventricular CMs in vitro and ventricular myocardium in vivo. Our results may lead to an alternative or a supplemental method for correcting defects in cardiac impulse generation, such as cell-based pacemakers.

Does the Functional Efficacy of Skeletal Myoblast Transplantation Extend to Nonischemic Cardiomyopathy?

BACKGROUND

The benefits of skeletal myoblast (SM) transplantation on infarcted myocardium have been investigated extensively; however, little is known about its effects in nonischemic cardiomyopathy models. To address this issue, we tested SM transplantation in CHF147 Syrian hamsters, a strain characterized by a delta-sarcoglycan deficiency that phenotypically features the human setting of primary dilated cardiomyopathy.

METHODS AND RESULTS

Cell culture techniques were used to prepare approximately 5x10(6) muscle cells from autologous tibialis anterior muscle, of which 50% were SMs (desmin staining). The cells were injected in 6 sites across the left ventricular wall (n=14). Control animals (n=12) received equivalent volumes of culture medium. Left ventricular systolic function was assessed in a blinded fashion from 2D echocardiographic left ventricular fractional area change, before transplantation, and 4 weeks later. Explanted hearts were processed for the detection of myotubes and quantification of fibrosis. Baseline functional data did not differ between the 2 groups. Four weeks after transplantation, 6 of the 10 surviving grafted hamsters were improved compared with 0 of the 8 survivors of the control group. This translated into a 6% decrease in fractional area change in controls compared with a 24% increase in cell-transplanted hamsters (P=0.001). Engrafted myotubes were consistently detected in all SM transplanted hearts by immunohistochemistry, whereas fibrosis was not worsened by cell injections.

CONCLUSIONS

These data suggest that the functional benefits of SM transplantation might extend to nonischemic cardiomyopathy.

Communication between interstitial cells of Cajal and gastrointestinal muscle

Interstitial cells of Cajal (ICC) pace gastrointestinal muscle by initiating slow waves in both muscle layers and appear to be preferred sites for reception of neurotransmitters. ICC of the myenteric plexus (ICC-MP) pace stomach and small intestine, while intramuscular ICC (ICC-IM) receive nerve messages. Recently, ICC-IM have been found to provide regenerative responses to and amplification of pacing messages from ICC-MP, at least in some systems. This review will examine the assumption that gap junctions provide low-resistance contacts for pacing. Structural and functional evidence will be evaluated. Structural, theoretical and experimental difficulties with the gap junctions hypothesis for pacing will be considered. So far little direct evidence about the role of gap junctions in neurotransmission exists, although a structural basis exists. Alternate possibilities for transmission of ICC pacing and neural messages will be examined and suggestions for future research made.

Transverse propagation of action potentials between parallel chains of cardiac muscle and smooth muscle cells in PSpice simulations

BACKGROUND

We previously examined transverse propagation of action potentials between 2 and 3 parallel chain of cardiac muscle cells (CMC) simulated using the PSpice program. The present study was done to examine transverse propagation between 5 parallel chains in an expanded model of CMC and smooth muscle cells (SMC).

METHODS

Excitation was transmitted from cell to cell along a strand of 5 cells not connected by low-resistance tunnels (gap-junction connexons). The entire surface membrane of each cell fired nearly simultaneously, and nearly all the propagation time was spent at the cell junctions, the junctional delay time being about 0.3-0.5 ms (CMC) or 0.8-1.6 ms (SMC). A negative cleft potential (Vjc) develops in the narrow junctional clefts, whose magnitude depends on the radial cleft resistance (Rjc), which depolarizes the postjunctional membrane (post-JM) to threshold. Propagation velocity (theta) increased with amplitude of Vjc. Therefore, one mechanism for the transfer of excitation from one cell to the next is by the electric field (EF) that is generated in the junctional cleft when the pre-JM fires. In the present study, 5 parallel stands of 5 cells each (5 x 5 model) were used.

RESULTS

With electrical stimulation of the first cell of the first strand (cell A1), propagation rapidly spread down that chain and then jumped to the second strand (B chain), followed by jumping to the third, fourth, and fifth strands (C, D, E chains). The rapidity by which the parallel chains became activated depended on the longitudinal resistance of the narrow extracellular cleft between the parallel strands (Rol2); the higher the Rol2 resistance, the faster the theta. The transverse resistance of the cleft (Ror2) had almost no effect. Increasing Rjc decreases the total propagation time (TPT) over the 25-cell network. When the first cell of the third strand (cell C1) was stimulated, propagation spread down the C chain and jumped to the other two strands (B and D) nearly simultaneously.

CONCLUSIONS

Transverse propagation of excitation occurred at multiple points along the chain as longitudinal propagation was occurring, causing the APs in the contiguous chains to become bunched up. Transverse propagation was more erratic and labile in SMC compared to CMC. Transverse transmission of excitation did not require low-resistance connections between the chains, but instead depended on the value of Rol2. The tighter the packing of the chains facilitated transverse propagation.

Activation of intestinal smooth muscle cells by interstitial cells of Cajal in simulation studies

Activation of a two-dimensional sheet network (5 parallel chains of 5 cells each) of simulated intestinal smooth muscle cells (SMCs) by one interstitial cell of Cajal (ICC) was modeled by PSpice simulation. The network of 25 cells was not interconnected by gap-junction channels; instead, excitation was transmitted by the electric field that develops in the junctional clefts (JC) when the prejunctional membrane fires an action potential (AP). Transverse propagation between the parallel chains occurs similarly. The ICC cell was connected to cell E5 of the network [5th cell of the 5th (E) chain] via a high-resistance junction. The stimulating current, applied to the ICC cell interior, was made to resemble the endogenous undershooting slow wave (I(SW)). An I(SW) of 2.4 nA (over a rise time of 4 ms) took the ICC cell from a resting potential (RP) of -80 mV to a membrane potential of -41 mV. The slow wave produced a large negative cleft potential in the JC (V(JC); ICC-E5). The V(jc) brought the postjunctional membrane of E5 to threshold, causing this cell to fire an AP. This, in turn, propagated throughout the SMC network. If the ICC cell was given an RP of -55 mV (like SMC) and a slow wave of 40 mV amplitude (I(SW) of 1.8 nA), it still activated the SMC network. This was also true when the ICC cell was made excitable (developing an overshooting, fast-rising AP). In summary, one ICC cell displaying a slow wave was capable of activating a network of SMC in the absence of gap junctions.

Cell coupling between ventricular myocyte pairs from connexin43-deficient murine hearts

Mice with cardiac-restricted inactivation of the connexin43 gene (CKO mice) have moderate slowing of ventricular conduction and lethal arrhythmias. Mechanisms through which propagation is maintained in the absence of Cx43 are unknown. We evaluated gap junctional conductance in CKO ventricular pairs using dual patch clamp methods. Junctional coupling was reduced to 4+/-2 nS (side-to-side) and 11+/-2 nS (end-to-end), including 21% of cell-pairs with no detectable coupling, compared with 588+/-104 nS (side-to-side) and 558+/-92 nS (end-to-end) in control cell-pairs. Voltage dependence of control gap junctions was characteristic of Cx43. CKO conductance showed increased voltage dependence, suggesting low-level expression of other connexin isoforms. From theoretical models, this degree of CKO coupling is not expected to support levels of conduction persisting in vivo, suggesting the possibility that there are additional mechanisms for maintained propagation when gap junctional conductance is severely reduced.

Propagation of action potentials between parallel chains of cardiac muscle cells in PSpice simulation

Propagation of action potentials between parallel chains of cardiac muscle cells was simulated using the PSpice program. Excitation was transmitted from cell to cell along a strand of three or four cells not connected by low-resistance tunnels (gap-junction connexons) in parallel with one or two similar strands. Thus, two models were used: a 2 x 3 model (two parallel chains of three cells each) and a 3 x 4 model (three parallel chains of four cells each). The entire surface membrane of each cell fired nearly simultaneously, and nearly all the propagation time was spent at the cell junctions, thus giving a staircase-shaped propagation profile. The junctional delay time between contiguous cells in a chain was about 0.2-0.5 ms. A significant negative cleft potential develops in the narrow junctional clefts, whose magnitude depends on several factors, including the radial cleft resistance (Rjc). The cleft potential (Vjc) depolarizes the postjunctional membrane to threshold by a patch-clamp action. Therefore, one mechanism for the transfer of excitation from one cell to the next is by the electric field (EF) that is generated in the junctional cleft when the prejunctional membrane fires. Propagation velocity increased with elevation of Rjc. With electrical stimulation of the first cell of the first strand (cell A1), propagation rapidly spread down that chain and then jumped to the second strand (B chain), followed by jumping to the third strand (C chain) when present. The rapidity by which the parallel chains became activated depended on the longitudinal resistance of the narrow extracellular cleft between the parallel strands (Rol2). The higher the Rol2 resistance, the faster the propagation (lower propagation time) over the cardiac muscle sheet (2-dimensional). The transverse resistance of the cleft had no effect. When the first cell of the second strand (cell B1) was stimulated, propagation spread down the B chain and jumped to the other two strands (A and C) nearly simultaneously. When cell C1 was stimulated, propagation traveled down the C chain and jumped to the B chain, followed by excitation of the A chain. Thus, there was transverse propagation of excitation as longitudinal propagation was occurring. Therefore, transmission of excitation by the EF mechanism can occur between myocardial cells lying closely parallel to one another without the requirement of a specialized junction.