Agonist-induced myosin phosphorylation during isometric contraction and unloaded shortening in airway smooth muscle

We measured myosin phosphorylation during isometric contraction at optimal length (Lo) and unloaded shortening induced by K(+)-depolarization, electrical stimulation, carbachol, histamine, and phorbol dibutyrate (PDB) in bovine trachealis. Peak myosin phosphorylation during unloaded shortening was lower than that during isometric contraction in response to all stimuli. The lower peak myosin phosphorylation during unloaded shortening appeared to be a stretch-sensitive response because myosin phosphorylation was either equally low or further reduced during the second unloaded shortening of preshortened tissues. Similar to peak myosin phosphorylation, steady-state myosin phosphorylation was also lower during unloaded shortening in carbachol-induced contractions. However, steady-state phosphorylation during unloaded shortening and isometric contraction were not significantly different in histamine- and PDB-induced contractions. Since the coupling between Ca2+ and myosin phosphorylation was not stretch sensitive, these results suggest the coexistence of stretch-sensitive and stretch-insensitive signal transduction mechanisms in the airway smooth muscle cell membrane, and the stretch-insensitive signal transduction mechanism might involve protein phosphorylation by protein kinase C.