Regulation of myosin light chain and phosphorylase phosphorylation in tracheal smooth muscle

The activity of phosphorylase kinase and myosin light chain kinase can be regulated by calcium-calmodulin or cyclic AMP. Since phosphorylation of the 20,000-dalton phosphorylatable light chain (P-light chain) of myosin may be involved in the regulation of contractile activity in smooth muscle, we examined the relationship between P-light chain phosphorylation, phosphorylase α formation, and isometric tension development during cholinergic and β-adrenergic stimulation of intact tracheal smooth muscle. The increase in isometric tension mediated by carbachol, a cholinergic agonist, was preceded by increases in both P-light chain and phosphorylase phosphorylation. Maximal increases in phosphorylation of both substrates occurred after 1 min and preceded the maximal development of isometric tension, which occurred at 3 min. After 1 min of incubation, the phosphate content of the P-light chain declined while isometric tension was maintained. However, phosphorylase α values remained elevated for up to 10 min, yet also subsequently declined during the maintenance of isometric tension. β-Adrenergic stimulation with 5 μM isoproterenol alone significantly increased phosphorylase α levels without affecting resting tension or P-light chain phosphorylation. Prior incubation with 5 μM isoproterenol also inhibited the rate and extent of both tension development and P-light chain phosphorylation during stimulation with 1 μM carbachol. However, further increases in phosphorylase α formation were evident. These data show that contraction of tracheal smooth muscle is accompanied by increases in phosphorylation of the P-light chain and phosphorylase. These increases are transient and are not sustained during the maintenance of isometric tension. β-Adrenergic stimulation concomitantly inhibits phosphorylation of the P-light chain and development of isometric tension.