Interaction of 28Mg with Ca and K in the smooth muscle of guinea-pig taenia coli.

1. The uptake of (28)Mg, and the total tissue content of Mg, Ca, Na and K have been determined in the smooth muscle of the guinea-pig taenia coli. The Mg content was 6.56 m-mole/kg fresh wt. immediately after dissection, falling slowly to 5.11 after 6 hr immersion in Krebs solution at 37 degrees C.2. The Mg content rose to 15.4 m-mole/kg fresh wt. during immersion in isotonic sucrose containing only MgCl(2). It was independent of the Mg(2+) concentration in this solution, but was depressed when K(+) or Ca(2+) ions were added.3. (28)Mg uptake showed three separate phases, extracellular, intermediate and slow. The size of the extracellular phase was proportional to the Mg(2+) concentration in the solution, but the size of the slow phase was constant. The size of the intermediate phase, exchanging with a half-time of a few minutes, was depressed when K(+) or Ca(2+) ions were added.4. The results are compatible with a competition between Mg in the intermediate phase of tracer exchange, and K(+) or Ca(2+) ions for fixed anionic sites in the tissue.