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1. Efflux experiments and longitudinal diffusion experiments were performed to determine the permeability of the surface membrane and of the nexal membrane of sheep and calf ventricular muscle to radioactive [1-¹⁴C]tetraethylammonium cations ([¹⁴C]TEA) at 25° C.

2. Efflux curves were analysed as the sum of three exponential processes with time constants for TEA loss of 2·7, 13·3, and 832 min.

3. The first component was attributed to the exchange of extracellular free TEA, the second to extracellular, probably mucopolysaccharide bound TEA, and the third to intracellular free TEA.

4. From the decay of the first phase the tortuosity factor for extracellular radial movement of TEA ions was calculated as 1·59.

5. The permeability of the surface membrane to [¹⁴C]TEA was estimated as 6·06 x 10^-8 cm/sec.

6. A tenfold change of [K]_o from 5·4 to 54 mM resulted in a 2·75-fold decrease of the permeability of the surface membrane; a fifteenfold change (5·4 to 81 mM) resulted in a 4·24-fold decrease.

7. From longitudinal non-steady state diffusion experiments the permeability of the nexal membrane to [¹⁴C]TEA turned out to be 1·27 x 10^-3 cm/sec.

8. The permeability of the nexal membrane to TEA is 21,000 times larger than that of the surface membrane. This finding is consistent with the concept of low resistance pathways between myocardial cells.

9. From a comparison of the dimensions of radiopotassium, [¹⁴C]TEA and Procion Yellow, and the study of their diffusional properties, the pores within the nexal membrane of the intercalated disks are deduced to have a diameter of somewhat more than 10 Å.

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