Membrane potential and ion content in the smooth muscle of the guinea-pig's taenia coli at different external potassium concentrations.

1. The relation between the membrane potential and the intracellular ion concentration of the taenia coli was studied in solutions with different external potassium concentrations.2. Isotonic solution in which the sum of NaCl+KCl was constant did not produce swelling of the smooth muscle of taenia coli, even at high [K](o), and did not change the intracellular chloride concentration.3. Addition of an equivalent amount (118 mM) of NaCl or KCl to Krebs solution produced the same loss of water.4. Solutions in which the product [K][Cl] was kept constant decreased the internal chloride concentration so that the chloride equilibrium potential became temporarily positive and returned only after 2 hr to a negative value.5. From these results and the finding that even in normal Krebs solution E(Cl) was about 35 mV more positive than the membrane potential, it is concluded that the chloride ions are not passively distributed. The potassium equilibrium potential is the main factor in the generation of the membrane potential. The large discrepancy between the two can be explained by the non-passive chloride distribution and by the sodium permeability of the membrane. The maintenance of the normal cell volume in solutions with constant sum of NaCl+KCl is explained by the constant intracellular chloride concentration.