DIFFERENTIAL-EFFECTS OF A K-CHAIN PHOSPHORYLATION IN RELAXATION OF ENDOTHELIN-1-CONTRACTED TRACHEAL SMOOTH-MUSCLE( CHANNEL AGONIST AND CA2+ANTAGONISTS ON MYOSIN LIGHT)

Smooth muscle contraction and relaxation are generally considered to b e associated with phosphorylation and dephosphorylation of the 20-kDa regulatory myosin light chain (LC(20)). Thus, contractions of lamb tra cheal smooth muscle induced by Bay K 8644 and relaxed by calcium chann el blockers (verapamil, D-600 and nitrendipine) are accompanied by an increase and decrease, respectively, of LC(20) phosphorylation. Simila rly, endothelin-1 (ET-1) induces a sustained contraction, which is cou pled with elevated LC(20) phosphorylation and reversed by LC(20) depho sphorylation after application of a potassium channel agonist (EMD 526 92). In contrast, calcium channel blockers relax ET-1-induced contract ion without any dephosphorylation of myosin light chains (MLC), sugges ting that MLC phosphatase is inhibited in this case. Obviously, MLC de phosphorylation is not a prerequisite for smooth muscle relaxation. Th e variable relationship between MLC phosphorylation and force during r elaxation suggests that there are mechanisms other than MLC phosphoryl ation that are important for regulation of contraction and relaxation in smooth muscle.