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

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.