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1. The intracellular K⁺, Cl^- and Na⁺ concentrations in the taenia coli cells of the guinea-pig have been estimated from the total ion content or the extrapolated intracellular tracer content, the sorbitol space and the dry wt./wet wt. ratio.

2. The exchange of K⁺, Cl^- and Na⁺ was studied by following the uptake and the efflux of these ions with radioactive isotopes. The following efflux values have been calculated: m_K, 4 p-mole.cm^-2.sec^-1; m_Cl, 8·4 p-mole.cm^-2.sec^-1 and m_Na, 7·2 p-mole.cm^-2.sec^-1. These flux values agree well with the influx values, obtained under the same experimental conditions.

3. The slowness of diffusion in the extracellular space reduces the Na flux by about 2·5% and the K flux by about 30%. A correction factor of 1·3 has to be introduced to obtain the true K flux.

4. The values for the permeability constants calculated by the constant field assumptions are for P_K, 11 x 10^-8 cm/sec; P_Cl, 6·7 x 10^-8 cm/sec and for P_Na, 1·8 x 10^-8 cm/sec. The introduction of these values and of the ion concentrations in the Goldman equation gives a resting potential of -37 mV.

5. One of the possible explanations for the discrepancy between the measured resting potential and the calculated one, is that the resting potential of these smooth muscle cells is partly a diffusion potential and partly due to the operation of an electrogenic Na pump.

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