Dissociation between myosin phosphorylation and shortening velocity in canine trachea

The relationship between glycogen phosphorylase activity (an index of cytosolic Ca2+ content), myosin light-chain phosphorylation, isotonic shortening velocity, and isometric tension was examined in canine trachealis. Responses were measured in tracheal strips contracted with various concentrations of methacholine or K+. Both agonists produced prolonged and concentration-dependent increases in isometric tension that reached 90% of the plateau level within 1 (methacholine) to 5 (K+) min and remained stable over 60 min. In contrast to the monotonic increase in isometric tension, shortening velocity reached a maximum almost immediately (12-48 s) after the addition of either methacholine or K+ and then declined over time to a steady-state level that was 25-40% of the peak. Phosphorylase activity also increased transiently, reaching a maximum 1-2 min after the addition of either agonist before declining to near-basal levels over the 60-min observation period. Unlike the increases in shortening velocity and phosphorylase activity, agonist-induced myosin phosphorylation was not markedly transient. Moreover, regardless of the contractile agonist used, no correlation was found between myosin phosphorylation and shortening velocity when these parameters were compared at corresponding time points. This suggests that myosin phosphorylation is not the sole determinant of shortening velocity in canine trachealis.