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.

No abstract

No abstract