ACTIN-SEVERING AND CA2+-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 fluorecence 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 mu M 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 mu M Ca2+, which inhibits the above contraction, was pultaceous with the actin filaments absent, indicating severing of actin filaments by a Ca2+-activated system, such as gelsolin.