Thick myofilaments in contracted and relaxed mammalian smooth muscle cells

Strips of Taenia coli from guinea pigs were incubated under physiological conditions in an organ bath. Contractile activity was monitored with an isometric force transducer and resting tension was adjusted with a micrometer drive. Strips in different functional states were fixed for subsequent examination in the electron microscope by the addition of glutaraldehyde to the organ bath. The density-distribution of thick myofilament profiles was obtained for relaxed and contracted strips. Thich myofilaments appear to be present in equal numbers in cells of spontaneously active, relaxed, and contracted strips.