Actin-severing and Ca(2+)-induced reversal of smooth muscle contraction that is independent of Ca2+.

1. Intracellular actin filament organization of gastric smooth muscle cells of the guinea pig in primary culture was examined with rhodamine-labelled phalloidin using a confocal laser fluorescence microscope. 2. The resting cells, both in the presence and absence of Ca2+, showed an even distribution of microfilamentous actin fibers. 3. The characteristic image of the stimulated cells with 10 microM acetylcholine in the presence of 1.8 mM Ca2+ was that the actin filaments were located only on the periphery of the cell. 4. The characteristic image of the cells stimulated as above, but in the absence of Ca2+, was that the actin filaments were unevenly distributed in the cell. 5. The characteristic image of the cells stimulated in the presence of 1 microM Ca2+, which inhibits the above contraction, was pultaceous with the actin filaments absent, indicating severing of actin filaments by a Ca(2+)-activated system, such as gelsolin.