Intercellular coupling in frog heart muscle

1. Passive electrical parameters of bullfrog atrial trabeculae were measured in a single gap arrangement. Attention was focussed on the resistance of internal longitudinal pathway. The influence of external Ca²⁺ depletion was tested using EGTA as chelating agent.
2. Morphometry of trabeculae, fine structure of junctional complexes, and distribution of membrane-bound Ca were investigated by light and electron microscopic methods.
3. The specific internal resistance to longitudinal current flow was 523 Ωcm with normal Ringer as perfusing fluid and 1140 Ωcm in EGTA-containing solution. These values are considered to represent the sum of myoplasmic and junctional resistivity.
4. Morphometrical studies indicated an interstitial space of 12%, a mean cell length of 358 μm, and a mean cell diameter of 3.2 μm.
5. In freeze-fractured preparations junctional structures were observed in the form of “atypical gap junctions” consisting of 10 nm particles arranged in a circular or linear array. The number of gap junctions was estimated to range between 20 and 50/cell which is equivalent to a junctional area of 0.01 or 0.03% of total surface area. A mean number of 55 particles/gap junction was calculated. After 20 min of exposure to EGTA the majority of junctional complexes were converted to clusters; the number of particles/gap junction was not significantly altered.
6. The fluorescent dye CTC was used as a probe for membrane-bound Ca of isolated living cells. In normal Ringer a strong fluorescence was seen at the cell surface and in different intracellular compartments. With EGTA both superficial and internal fluorescence disappeared completely.
7. From a combination of electrical and morphometrical data the resistance of intercellular junctions was calculated. Under normal conditions the specific resistance of junctional membrane amounted to 0.4 Ωcm² and the resistance of an individual connexon was of the order of 10¹¹ Ω. With EGTA, the respective values were increased by about 230%. The mechanism underlying this depression of junctional conductance is not clear. It seems not related to a rise of cytoplasmic free Ca²⁺. The EGTA-induced increase in internal resistance was reflected by a decrease of the length constant of a bundle.
8. The nature of “atypical gap junctions” and their relation to tight junctions are discussed. It is concluded that the junctions observed in frog atrial muscle are analogous to gap junctions of insect or mammalian cells in spite of the different size and arrangement of the particles.
9. A theoretical model is presented for the electrical behaviour of a bundle in a single gap arrangement. The bundle is simulated by a partially isolated cable of finite length. Voltage distribution along the cable in response to a d.c. pulse applied externally is described as a function of distance and time.