Intercalated disk

Interaction parameters for polymer blends, 20 322 in surfactant adsorption, 24 138 Interaortic balloon pump, 3 746 Intercalated disks, myocardium, 5 79 Intercalate hybrid materials, 13 546-548 Intercalation adducts, 13 536-537 Intercalation compounds, 12 777 Intercritical annealing, 23 298 Interdiffusion, 26 772 Interdigitated electrode capacitance transducer, 14 155 Interesterification, 10 811—813, 831 Interest expense, 9 539 Interface chemistry, in foams, 12 3—19 Interface metallurgy materials, 17 834 Interfaces defined, 24 71... 

Fig. 2. Phase contrast microscopy of rabbit cardiac muscle. Note the Glanzstreifen with the intercalated disk (arrow), x 1,000. 

The spatial structure of the channel has been investigated for a long time. In the beginning, light microscopists described intercalated disks which appeared as bands transverse to the longitudinal axis of the cardiac muscle fiber [Eberth, 1866]. With modem phase contrast microscopes they can easily be seen as shown in figure 2. 

These bands were a matter of discussion for a long time until in 1954, for the first time, Sjostrand and Andersson used electron microscopy to investigate intercalated disks in ultrathin osmium tetroxide-fixed sections of the mouse heart revealing that the disks were indeed transverse cell boundaries. Subsequently, several investigators reproduced their finding [Lindner, 1957 Moore... 

Fig. 9. Distribution of the gap junctions, desmosomes and fascia adherens in an intercalated disk of a cardiomyocyte as assessed by electron microscopy of freeze-fractured rat and rabbit hearts according to Severs [1990]. 

The surface of the intercalated disk is occupied to 5.7 + 0.6% by gap junctions (canine atrium) [Spira, 1971], 3.3% (right bundle, calf) [Arluk and Rhodin, 1974], or even 12.7-15.1% in canine left ventricular subepicardial myocardium [Hoyt et al., 1989]. The rest of the intercalated disk is made of fascia adherens and desmosomes. In the crista terminalis of the canine heart the gap junction profile length has been estimated to be in the order of 3.2-3.8 pm/100 pm intercalated disk length with 11-12 gap junctions/100 pm intercalated disk length and a mean gap junction profile length of about 0.3 pm [Saffitz et al., 1994]. 

In patients with cardiac hypertrophy from chronically pressure-loaded human left ventricles due to aortic valve stenosis, a general reduction in gap junction surface area per unit cell volume by about 40% (0.0031 versus 0.0051 pm2/ pm3) has been observed [Peters et al., 1993]. The gap junctions in the pathological tissue were larger than normal. The estimated gap junction content per cell was reduced [Peters et al., 1993]. A reduction by 30% in the gap junction surface per cell was observed [Peters, 1996]. However, the number of intercalated disks per myocyte and the mean density of packing of connexons at freeze-fracture in these hearts remained unchanged as compared to control hearts. 

Macdonald RL, Hsu D, Mann JE, Sperelakis N An analysis of the problem of K+ accumulation in the intercalated disk clefts of cardiac muscle. J Theor Biol 1975 52 455-474. 

Severs NJ The cardiac gap junction and intercalated disk. Int J Cardiol 1990 26 137-173. 

Sjostrand FS, Andersson E Electron microscopy of intercalated disks of cardiac muscle tissue. Experientia... 

Weingart R The permeability to tetraethylammonium ions of the surface membrane and the intercalated disks of sheep and calf myocardium. J Physiol (Lond) 1974 240 741-762. 

Soonpaa MH, Koh GY Klug MG, Reid LJ. Formation of nascent intercalated disks between grafted fetal cardiomyocytes and host myocardium. Science 1994 264(5155) 98-101. 

Fig. 3. Immunohistochemical reaction for /3-catenin. (A) Positive reaction in intercalated disk in the normal human heart. (B) In cardiomyocytes bordering a scar after MI, /3-catenin is disorganized, fragmented, and located in the cytoplasm.

Figure 6.2. Ultrastructure of heart muscle, a Light microscopy. Striations mn perpendicularly to the cell longitudinal axis. Junctions between individual cells ( intercalated disks ) are visible as slightly lighter bands they contain numerous gap junctions, b Electron microscopy (EM), and correlation to hght microscopy (LM) and spatial arrangement of actin and myosin.

Figure 2. Representative examples of total Cx43 (red) and non-phosphorylated Cx43 (green) from a non-prcconditioncd heart (left side) and a preconditioned heart (right side). While in non-preconditioned hearts the density of non-phosphorylated Cx43 at the intercalated disks increased during 85-90 min ischemia (yellow color), the density remained unchanged in preconditioned hearts.

Cohen SA 1996 Immunocytochemical localization of rHl sodium channel in adult rat heart atria and ventricle. Presence in terminal intercalated disks. Circulation 94 3083-3086... 

As with the normal cardiac conducting system and its propagation of impulses through the His Purkinje network, electrostimulation by an artificial cardiac pacemaker depends on the depolarization of a single or a group of myocyte cell membranes which can then act as pacemaker cells. In order for these cells to depolarize, the electric field of the applied artificial pacemaker stimulus must exceed a threshold voltage. This initiates a complex cascade of ionic currents both in and out of the cell membrane referred to as the action potential. The impulse or wave of depolarization then propagates away from the site of stimulation from cell to cell across gap junctions or intercalated disks, which with normal cells provide very low resistance to depolarization. 

In this way, the impulse penetrates all areas of either the atria or ventricles and thus initiates contraction. In pathological situations, such as myocardial ischemia, propagation may slow down because of a rise in resistance in the intercalated disks. 

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