Calculation of the membrane potential in smooth muscle cells of the guinea‐pig's taenia coli by the Goldman equation

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Abstract
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: mK, 4 p‐mole.cm−2.sec−1; mCl, 8·4 p‐mole.cm−2.sec−1 and mNa, 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 PK, 11 × 10−8 cm/sec; PCl, 6·7 × 10−8 cm/sec and for PNa, 1·8 × 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.