Light and electron microscopic studies regarding cell contractility and cell coupling in light sensitive smooth muscle cells from the isolated frog iris sphincter.

(1) In light microscopical studies of living isolated frog irises, it was found that the maximal areas of experimentally light induced contractions in the m. sphincter pupillae were located beneath small illuminated regions. There were no visible contractions of muscle cells outside the illuminated areas. It was shown that exposure to light could directly cause contractions of isolated single sphincter muscle cells. (2) Junctional structures of the iris sphincter cells were studied by means of thin sections and freeze fracture electron microscopy. Intermediate junctions, a few focal tight junctions and occasional small gap junctions were identified. Pit containing intramembranous particles which resemble gap junctions connexons were found in large numbers, dispersed over the plasmalemmas of sphincter muscle cells. From these physiological and morphological observations, it is concluded that sphincter muscle cells of the frog iris may be coupled via gap junctions, but that the cell coupling is not sufficiently extensive to form the basis for a functional syncytium.