POLYMERIZATION OF MYOSIN ON ACTIVATION OF RAT ANOCOCCYGEUS SMOOTH-MUSCLE

The in vivo state of assembly of myosin in vertebrate smooth muscle is controversial. In vitro studies on purified smooth muscle myosin show that it is monomeric (10S) under relaxing conditions and filamentous under contraction conditions. Electron microscopic and antibody labell ing studies of intact smooth muscles, on the other hand, suggest that myosin is filamentous in the relaxed as well as the contracting state and that 10S myosin occurs only in trace amounts. However, birefringen ce, conventional EM and X-ray diffraction evidence suggests that in ce rtain smooth muscles in vivo (e.g. rat anococcygeus), while myosin fil aments exist in the relaxed state, their number increases on contracti on. Here, we have used low temperature electron microscopic techniques (rapid freezing followed by freeze-substitution), which preserve labi le components in close to their in vivo state, to detect any change in filament number on contraction. The results from rat anococcygeus hav e been compared with those from guinea pig taenia coli, in which other techniques have revealed no change in filament number. In the anococc ygeus, we find evidence for a 23% increase in filament density in tran sverse sections of contracting muscle compared with relaxed muscle. In the taenia coli we find no change. These results are in qualitative a greement with earlier findings. They provide evidence for polymerizati on of myosin in contracting rat anococcygeus, and suggest that this pr ocess is subtle and occurs only in some smooth muscles.