Actin polymerization stimulated by contractile activation regulates force development in canine tracheal smooth muscle

1. The role of actin polymerization in the regulation of smooth muscle cont ractility was investigated in canine trachealis muscle strips. The effect o f contractile activation on the content of monomeric globular (G)-actin was estimated by the method of DNase I inhibition. The G-actin content was 30% lower in extracts of muscle strips activated with 10(-4) M acetylcholine ( ACh) than in extracts from unstimulated muscle st rips. The decrease in G-a ctin in response to contractile stimulation was prevented by latrunculin-A, an agent that prevents actin polymerization by binding to G-actin monomers . 2. The inhibition of actin polymerization by latrunculin-A markedly depress ed force development in response to ACh but had no effect on ACh-induced my osin light chain (MLC) Phosphorylation. Latrunculin also suppressed the len gth sensitivity of force during ACh-induced isometric contractions. The act in-capping agent cytochalasin-D also markedly inhibited force and caused on ly a slight decrease in MLC phosphorylation. Cytochalasin-D also inhibited force in alpha-toxin-permeabilized muscle strips that were activated either by Ca2+ or by ACh at constant pCa. No disorganization of smooth muscle cel l ultrastructure was detected by electron microscopy or by immunofluorescen ce microscopy of muscles treated with either agent. 3. The results suggest that the polymerization of actin is stimulated by th e contractile activation of tracheal smooth muscle and that this actin poly merization contributes directly to force development. In addition, actin fi lament remodelling contributes to the length sensitivity of tracheal smooth muscle contractility.