Modeling electric field transfer of excitation at cell junctions

The electric field (EF) model was first developed on a "breadboard" using physical electric components (e.g., resistors, capacitors, batteries) and was then modeled mathematically by a series of differential equations and matrix equations and simulated on a large computer (CDC-6400). The results obtained by the two methods agreed very closely. However, these two methods of analysis are quite cumbersome. Therefore, in order to simplify the EF simulation, we wanted to model it on the PSpice program. In this article we discuss how we succeeded in demonstrating transmission of excitation from cell to cell in cardiac muscle and smooth muscle based on EF transmission at the cell junctions.     

Intracellular application of calmidazolium increases Ca2+ current through activation of protein kinase A in cultured vascular smooth muscle cells

This study compared markers of the metabolic processes occurring in male and female adolescent triathletes from two age groups (over 15 years of age [O15] and under 15 years of age [U15]) during a laboratory based duathlon. Participants were tested on three separate occasions; two peak VO2 tests on a treadmill and cycle ergometer, and a third session involved a simulated duathlon (2 km run, 12 km ride and 4 km run for the O15 group or 1 km run, 8 km ride and 2 km run for the U15). Data collection included performance speed, cardiorespiratory responses and blood borne markers of exercise metabolism. The performance speeds selected by the two age groups did not differ. The mean relative percentage of VO2peak at which subjects participated were 79+/-3, 77+/-4%, for the O15 males and females, and 71+/-5 and 82+/-2%, for the U15 males and females, respectively. While the plasma metabolites of ammonia [NH3] and lactate [La] were not different between age groups and sex (p>0.05) there were however, higher concentrations recorded during the cycling phase when compared with the running phases (p < 0.05). The respective mean concentrations for NH3 and La were 80.5+/-5.6 microM, and 4.9+/-0.3 microM for cycling, and 56.3+/-2.7 microM, and 2.7+/-0.2 microM for the combined running phases.     

Angiotensin II stimulation of Ca2+-channel current in vascular smooth muscle cells is inhibited by lavendustin-A and LY-294002

Angiotensin II (AngII) is coupled to several important intracellular signaling pathways, and increases intracellular Ca2+. In vascular smooth muscle (VSM) cells, AngII is known to activate enzymes such as tyrosine protein kinase (Tyr-PK), phospholipase C (PLC), protein kinase C (PKC), and phophatidylinositol-3-kinase (PI-3-K). A non-receptor Tyr-PK, pp60(c-src), and PKC have been reported to stimulate the Ca2+ channels in VSM cells. However, less is known about AngII action on the voltage-gated Ca2+ channels. The Ca2+-channel currents of a cultured rat aortic smooth muscle cell line, A7r5, were recorded using whole-cell voltage clamp. Application of 50 nM AngII significantly increased the amplitude of Ba2+ currents through the voltage-gated Ca2+ channels (IBa) by 34. 5+/-9.1% (n=10) within 1 min. In the presence of lavendustin-A (5 microM), a selective inhibitor of Tyr-PK, AngII failed to stimulate IBa (n=5). AngII stimulation of IBa was also prevented by (5 microM) LY-294002, an inhibitor of PI-3-K (n=5). In contrast, H-7 (30 microM), an inhibitor of PKC, did not prevent the effect of AngII on IBa (n=6). These results suggest that AngII may stimulate the Ca2+ channels of VSM cells through Tyr-PK and PI-3-K under conditions that probably exclude participation of PK-C.     

Impedance analysis applicable to cardiac muscle and smooth musclebundles

An electrical equivalent circuit was constructed to represent a chain of five myocardial cells in a cardiac muscle bundle with various degrees of cell-to-cell coupling, and an impedance analysis was performed. The impedance across the entire network was measured at frequencies ranging from 10(1) to 10(6) Hz. The Bode plots were nearly superimposable for 1, 10, and 100 tunnels; for 10(3), 10(4), and 10(5) tunnels, the absolute zeta at 10 Hz was lower: e.g., 9.82 M omega for 1 tunnel compared to 6.64 M omega for 10(5) tunnels. The delta zeta 1/2 values were shifted to the left in the well-coupled cases: e.g., for 1 tunnel, f1/2 was 37.8 kHz, and for 10(5) tunnels, f1/2 was 1.2 kHz. For high coupling, the Bode plots contained a double component due to the end membranes. When Ro was increased by eight times, zeta increased by 7.47 fold (for 1 tunnel, 10 Hz), and by 3.72 fold (for 10(5) tunnels, 10 Hz). Raising Ro to x 12, x 100, and x 1000 produced a further and further shift to the left of the Bode plots. The total tissue resistivity (Rt) increased as a function Ro. Thus, in low coupling cases, almost all of the applied current passes through the interstitial space; e.g., at 1 tunnel (10 Hz), 1.0% of the current passes through the cell pathway (Rcell). The ratio of impedances at 10 kHz to 10 Hz (zeta 10kHz/zeta 10Hz) decreased with increasing tunnels (for Ro x 1). The ratio of resistivities at Ro x 8 to Ro x 1 (Rt'/Rt) was 7.47 for 1 tunnel. In contrast, the ratio at 10(5) tunnels was 3.73. It is concluded that it is difficult to determine the degree of cell coupling from such impedance analysis, unless the same tissue can be used for its own control, i.e., before and after a large change in cell coupling is introduced.     

Role of the steady-state Na+ channel current in pacemaker depolarizations in young embryonic chick ventricular myocytes

Thresholds to noxious mechanical and thermal stimulation were measured in 6 groups of sheep prior to induction of anaesthesia and subsequently for a period of 2 h in the post-anaesthetic period. Groups 1-4 were anaesthetised using thiopentone and underwent ventral midline laparotomy. Four animals (group 5) underwent anaesthesia but not surgery, and a further 6 sheep (group 6) undergoing surgery were anaesthetised using ketamine. Groups 1-3 were intravenously administered the following drugs intra-operatively: flunixin meglumine, carprofen and buprenorphine, respectively. Groups 4-6 received no additional treatment. Thresholds to the mechanical test were not changed in the post-anaesthetic period for any group. There was a significant reduction in the responses to thermal stimulation after surgery for sheep in group 4 (45 and 60 min), while sheep in group 2 had thresholds to thermal stimulation greater than those recorded in the remaining groups at all time points post-operatively. Responses to thermal stimulation in sheep undergoing anaesthesia but not surgery (group 5) were unaltered during the 2 h recording period after anaesthesia ended. These data indicate that abdominal surgery induces thermal but not mechanical hyperalgesia in sheep, which appears to be centrally mediated. Moreover, the absence of mechanical hyperalgesia raises the possibility that central changes in noxious information processing may not be detected using mechanical stimuli in the same time course as thermal stimuli.     

Electric field interactions between closely abutting excitablecells

This review article summarizes some of the electrophysical evidence and morphological evidence against the hypothesis that the myocytes of cardiac muscles and visceral smooth muscles are profusely interconnected by low-resistance pathways (e.g., tunnels or gap-junction channels), which would give rise to a long length constant. Instead, propagation of the action potential (AP) is discontinuous, with a substantial junctional delay time at the cell junctions. Since the entire surface membrane of each cell becomes excited nearly simultaneously, a plot of propagation time versus distance (along a strand of cells) exhibits a typical staircase shape. This article demonstrates that the electric field that develops in the narrow junctional cleft (negative cleft potential) when the prejunctional membrane (pre-JM) fires an AP acts to depolarize the post-JM to its threshold. This mechanism, by itself, can account for transmission of excitation from cell to cell, but accessory mechanisms that act additively include K⁺ accumulation in the junctional clefts, gap-junction channels, and capacitive coupling     

Propagation down a chain of excitable cells by electric field interactions in the junctional clefts: effect of variation in extracellular resistances, including a "sucrose gap" simulation

An electric field model for electrical transfer of excitation between contiguous excitable cells has been further developed by expanding the model to a chain of six cells, and examining the effect of changing the external resistances on propagation velocity. In this model, there is no requirement for low-resistance connections between the cells, and the major assumption is that the pre-and postjunctional membranes are ordinary excitable membranes. The electric field that develops in the narrow junctional cleft between contiguous cells during the rising phase of the action potential in the prejunctional membrane acts to depolarize the postjunctional membrane to threshold. Propagation occurred down the entire chain of cells at a constant velocity of about 17.1 cm/s. Raising the extracellular resistances (ROL and ROR) along the entire chain up to fourfold slowed propagation only slightly. However, when the radial cleft resistance (RJC) was varied concomitantly, then there was a marked slowing of propagation velocity, e.g., to 3.3 cm/s in 4.0 X resistance. There was an optimal RJC value for peak velocity. The lowering of ROL and ROR up to eightfold has almost no effect on velocity. Raising RJC, ROL, and ROR for the middle two cells, up to 3 times the normal value slowed propagation in the "sucrose-gap" region; raising the resistance to 4 times or higher blocked propagation. Hence, the electric field model allows successful transmission of excitation down a long chain of cells, not connected by low-resistance tunnels, at a constant velocity, and propagation velocity is dependent particularly on RJC.     

Effect of variation in membrane excitability on propagation velocity of simulated action potentials for cardiac muscle and smooth muscle in the electric field model for cell-to-cell transmission of excitation

We have previously published several studies on the propagation of simulated action potentials (APs) of cardiac muscle and smooth muscle using the PSpice program. Those studies were done on single chains of five to ten cells in length to examine longitudinal propagation between the cells, either not connected by gap-junction (g.j) channels or connected by various numbers of channels. In addition, transverse propagation was examined between parallel chains (two to five chains) not connected by g.j. channels. In all those studies, the myocardial cells and smooth muscle cells (SMCs) were unintentionally somewhat hyperexcitable by virtue of the values inserted into the GTABLEs of the PSpice program. Because transmission of excitation from cell to cell occurred very well in the absence of g.j. channels, by virtue of the electric field (EF) generated in the narrow junctional clefts (negative cleft potential V/sub JC/), the present study was carried out, in which the cells were made hypo-excitable by altering the GTABLE values. Three levels of excitability of the cardiac cells and SMC were examined: 1) high; 2) intermediate; and 3) low. It was found that propagation of excitation, both longitudinally and transversely, can occur by the EF mechanism alone, even when the excitability of the cells was low. Therefore, the EF mechanism alone can account for propagation of excitation in cardiac muscles and smooth muscles that do not possess gap junctions. In those cases in which gap junctions do exist and are functioning, the EF mechanism would act in parallel and thereby increase the safety factor for conduction.     

Alteration in Sodium Channel Gate Kinetics of the Hodgkin-Huxley Equations Applied to an Electric Field Model for Interaction Between Excitable Cells

An electric field model for electrical transmission of excitation between adjacent myocardial cells, without the necessity of low-resistance connections between the cells, is further developed. The voltage dependency of the membrane conductances was modeled by the Hodgkin-Huxley equations. A major assumption of the model is that the pre-and postjunctional membranes are excitable. The bulk electrical properties (capacitance, conductivity, etc.) of the junctional membranes may be the same as those of the surface membranes. We showed that a modification of the dynamics of the fast sodium channel gates of the junctional membranes (controlled by the m and h parameters) can be employed as a mechanism to secure propagation of an impulse. The propagation velocity was determined primarily by the dynamics of the junctional membranes, and was essentially independent of the rate of rise of the surface units. Propagation at a constant velocity occurred when the electric field model was expanded to a chain of six cells. Thus, the electric field model, based on closely apposed and excitable junctional membranes, could account for propagation in cardiac muscle, and may apply under various physiological and pathophysiological conditions.