An interaction between potassium and sodium in the smooth muscle of the guinea-pig taenia coli.

1. The potassium content of the guinea-pig taenia coli was 72 m-mole K/kg fr. wt. after equilibration with normal Krebs-type solution at 35 degrees C in vitro.2. It fell to 13 m-mole K/kg fr. wt. when Ca(2+) and Mg(2+) were omitted from the bathing solution, but this fall was reversed in part when [Na(+)](o) was also reduced.3. The taeniae relaxed when Ca(2+) and Mg(2+) were omitted from the normal solution. However, they contracted if Na(+) was also omitted.4. Effects 2 and 3 may show some antagonism between sodium and divalent cations in smooth muscle.5. The extracellular space of the same muscles was measured with [(14)C]sorbitol. It was 440 ml./kg fr. wt. in normal solution.6. The uptake of (42)K was measured in the same muscles at the same time. An initial rapid exchange was followed within 2 min by a slow (half-time [unk] 50 min in normal solution) and presumably intracellular uptake of tracer.7. In normal solution the initial rapid phase of (42)K exchange corresponded to 3.0 m-mole K/kg fr. wt. A value of 2.6 m-mole K/kg fr. wt. would have been calculated from [K(+)](o) and the [(14)C]sorbitol space, and these estimates did not differ significantly.8. The [(14)C]sorbitol space fell slightly when [Ca(2+)](o), [Mg(2+)](o), and [Na(+)](o) were reduced, but the amount of rapidly exchanging potassium increased significantly reaching 4.6 m-mole K/kg fr. wt. in solutions from which Ca(2+), Mg(2+) and Na(+) were omitted. The [(14)C]sorbitol space only accounted for 2.4 m-mole K/kg fr. wt. under these conditions, a significantly smaller quantity.9. The observations have been interpreted on Wilbrandt & Koller's (1948) hypothesis that there may be a superficial anionic region in muscle cells. On this model the present results suggest that K(+) rather than Na(+) is favoured as a monovalent counter-cation in the taenia coli.