The influence of high potassium depolarization and acetylcholine on calcium exchange in the rat uterus.

Net and radioactive calcium movements were studied in the rat uterus during stimulation with acetylcholine and high potassium solutions. High potassium did not affect the efflux of intracellular Ca, but was able to release Ca from a small parallel Ca fraction which was believed to be located in the cell membranes. High potassium did markedly slow the influx of Ca and caused a net calcium efflux. Acetylcholine had no effect on calcium movements in polarized myometrium, but it increased the Ca influx in depolarized uteri. Ca taken up during contraction exchanged more slowly during subsequent efflux than Ca taken up at rest. The results were interpreted as supporting the hypothesis that myometrial contraction is induced by a release of calcium from the inside of the cell membrane and the endoplasmic reticulum, and relaxation follows the removal of ionic cytoplasmic calcium by these same structures.