The sarcoplasmic reticulum (SR) was studied in the smooth muscles of rabbit main pulmonary artery, mesenteric vein, aorta, mesenteric artery, taenia coli, guinea pig mesenteric artery, and human uterus, and correlated with contractions of the smooth muscles in Cafree media . S R volumes, were determined in main pulmonary artery (5 .1 %), aorta (5%0), portal-anterior mesenteric vein (2 .2%), taenia coli (2%), and mesenteric artery (1 .8%) : because of tangentially sectioned membranes these estimates are subject to a correction factor of up to +50% of the values measured . Smooth muscles that contained a relatively large volume of SR maintained significant contractile responses to drugs in the virtual absence of extracellular calcium at room temperatures, while smooth muscles that had less SR did not . The unequal maximal contractions of main pulmonary artery elicited by different drugs were also observed in Ca-free, high potassium-depolarizing solution, indicating that they were secondary to some mechanism independent of changes in membrane potential or calcium influx . Longitudinal tubules of SR run between and are fenestrated about groups of surface vesicles separated from each other by intervening dense bodies . Extracellular markers (ferritin and lanthanum) entered the surface vesicles, but not the SR. The peripheral SR formed couplings with the surface membrane : the two membranes were separated by gaps of approximately 10 nm traversed by electron-opaque connections suggestive of a periodicity of approximately 20-25 nm . These couplings are considered to be the probable sites of electromechanical coupling in twitch smooth muscles . Close contacts between the SR and the surface vesicles may have a similar function, or represent sites of calcium extrusion . The presence of both thick and thin myofilaments and of rough SR in smooth muscles supports the dual, contractile and morphogenetic, function of smooth muscle .

A clonal cell line derived from a mouse neoplasm is described which shares many properties with smooth muscle . The cells have electrically excitable membranes capable of generating overshooting action potentials, and they contract both spontaneously and with electrical stimulation . They respond to the iontophoretic application of acetylcholine with a depolarizing response, and to norepinephrine with a hyperpolarizing response . Electron microscopy reveals that the cells have a morphology similar in many, but not all, respects to that of smooth muscle cells in vivo . The cells secrete soluble collagen-like molecules in addition to several proteins of undefined function . Finally, there is an increase in the specific activities of creatine phosphokinase and myokinase associated with increased cell density and the cessation of cell division.

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Thin (actin), thick (myosin) and interm ediate filaments are described in vertebrate smooth muscle. The thick filaments are present in relaxed, contracted, stretched and unstretched vertebrate smooth muscle and bear lateral projections suggestive of cross-bridges. The relatively regular thick filament lattice of the rabbit portal-anterior mesenteric vein can be aggregated by hypertonic solutions and excessive stretch. The interm ediate filaments are morphologically distinct and clearly not breakdown products of thick filaments.

The contractile response of turtle oviduct smooth muscle to acetylcholine after 30 min of incubation of muscles in Ca-free, 4 mm ethylene (bis) oxyethylenenitrilotetraacetic acid (EGTA) solutions at room temperature was greater than the contractile response after 30 min of incubation in the Ca-free medium at 37°C. Incubation in Ca-free solution at 37°C before stimulation with acetylcholine in Ca-free solutions at room temperature also reduced the contractile response, suggesting that activator calcium was lost from the fibers at a faster rate at higher temperatures . Electron micrographs of turtle oviduct smooth muscle revealed a sarcoplasmic reticulum (SR) occupying approximately 4% of the nucleus-and mitochondria-free cell volume . Incubation of oviduct smooth muscle with ferritin confirmed that the predominantly longitudinally oriented structures described as the SR did not communicate with the extracellular space . The SR formed fenestrations about the surface vesicles, and formed close contacts (couplings) with the surface membrane and surface vesicles in oviduct and vena caval smooth muscle ; it is suggested that these are sites of electromechanical coupling. Calculation of the calcium requirements for smooth muscle contraction suggest that the amount of SR observed in the oviduct smooth muscle could supply the activator calcium for the contractions observed in Ca-free solutions . Incubation of oviduct smooth muscle in hypertonic solutions increased the electron opacity of the fibers . A new feature of some of the surface vesicles observed in oviduct, vena caval, and aortic smooth muscle was the presence of approximately 10 nm striations running approximately parallel to the openings of the vesicles to the extracellular space . Thick, thin, and intermediate filaments were observed in turtle oviduct smooth muscle, although the number of thick filaments seen in the present study appeared less than that previously found in mammalian smooth muscles .The anatomical source of the calcium that activates smooth muscle contraction has not been established . In contrast, it is now generally accepted that in fast-striated muscles the sarcoplasmic reticulum is a storage site of activator calcium (e .g .