Electric field interactions between closely abutting excitable cells

Does ICC pacing require functional gap junctions between ICC and smooth muscle in mouse intestine?

We tested the hypothesis that interstitial cells of Cajal (ICC) pace longitudinal and circular muscle of mouse intestine through gap junctions. Carbenoxolone (10(-6), 10(-5), 10(-4) mol L(-1)), an inhibitor of gap junction conductance, was applied to segments of longitudinal or circular muscle with contractions driven by ICC after inhibition of nerve function by tetrodotoxin (10(-6) mol L(-1)) and L-NOARG (10(-4) mol L(-1)). Carbenoxolone concentration- and time-dependently inhibited the amplitude of contraction (0.2-1.5 g in controls) of segments of longitudinal muscle, but had no effect on the frequency of contractions (from 36-54 min). It also inhibited the amplitude of contractions of circular muscle segments and reduced the frequency slightly at 10(-4) mol L(-)1. Carbenoxolone inhibited tonic contractions of longitudinal but not circular segments to 60 mmol L(-1) KCl, suggesting that it directly inhibited contractions of longitudinal muscle. The responses to pacing by electrical field stimulation (40 V cm(-1), 50-100 ms, 1 Hz) after block of nerve function were reduced insignificantly in amplitude, and not in frequency in both longitudinal and circular segments. We conclude that it is likely that only gap junctions within circular muscle are involved in pacing of muscle by ICC. Carbenoxolone also has effects on muscle contractility in longitudinal muscle.

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.

Localization of sodium channels in intercalated disks modulates cardiac conduction

It is well known that the sodium current (I(Na)) and the degree of gap-junctional electrical coupling are the key determinants of action potential (AP) conduction in cardiac tissue. Immunohistochemical studies have shown that sodium channels (NaChs) are preferentially located in intercalated disks (IDs). Using dual immunocytochemical staining, we confirmed the colocalization of NaChs with connexin43 in cultures of neonatal rat ventricular myocytes. In mathematical simulations of conduction using the Luo-Rudy dynamic model of the ventricular AP, we assessed the hypothesis that conduction could be modulated by the preferential localization of NaChs in IDs. Localization of I(Na) at the ID caused a large negative potential in the intercellular cleft, which influenced conduction in two opposing ways, depending on the degree of electrical coupling: (1) for normal and moderately reduced coupling, the negative cleft potential led to a large overshoot of the transmembrane potential resulting in a decreased driving force for I(Na) itself (self-attenuation), which slowed conduction; (2) for greatly reduced coupling (<10%), the negative cleft potential induced by I(Na) in the prejunctional membrane led to suprathreshold depolarization of the postjunctional membrane, which facilitated and accelerated conduction. When cleft potential effects were not incorporated, conduction was not significantly affected by the ID localization of I(Na). By enhancing conduction through the establishment of cleft potentials, the localization of NaChs in IDs might protect the myocardium from conduction block, very slow conduction, and microreentry under conditions of greatly reduced coupling. Conversely, by supporting moderately slow conduction, this mechanism could also promote arrhythmias.