Biochemical events associated with activation of smooth muscle contraction

Biochemical events associated with activation of smooth muscle contraction were studied in neurally stimulated bovine tracheal smooth muscle. A latency period of 500 ms preceded increases in isometric force and myosin light chain phosphorylation. However, stimulation resulted in the rapid hydrolysis of inositol phospholipids as demonstrated by increases in inositol phosphates by 500 ms. Inositol trisphosphate increased 2-fold with no significant change in inositol tetrakisphosphate. The apparent activation state of myosin light chain kinase was assessed indirectly through measurements of the fractional activation of a second calmodulin-dependent enzyme, cyclic nucleotide phosphodiesterase. The fractional activation of cyclic nucleotide phosphodiesterase increased after neural stimulation to a maximal extent by 500 ms and remained at this level for at least 4 s. The monophosphorylation of myosin light chain increased after 500 ms and reached a maximum value by 2 s. Diphosphorylation also occurred but to a much lesser extent. Fractional activation of cyclic nucleotide phosphodiesterase and myosin light chain phosphorylation both decreased after 10 min continuous stimulation, although the force response remained at a maximal level. These observations demonstrate that inositol trisphosphate formation and activation of cyclic nucleotide phosphodiesterase (and hence most likely myosin light chain kinase) by calmodulin precede myosin light chain phosphorylation and that these events are sufficiently rapid to mediate the contractile response of neurally stimulated tracheal smooth muscle.