THE BIOCHEMICAL BASIS OF THE REGULATION OF SMOOTH-MUSCLE CONTRACTION

The primary signal for smooth-muscle contraction is an increase in sar coplasmic free Ca2+ concentration ([Ca2+](i)). This triggers activatio n of calmodulin-dependent myosin light-chain kinase, which catalyses m yosin phosphorylation, thereby activating crossbridge cycling and the development of force or contraction of the muscle cell. Restoration of resting [Ca2+](i) deactivates the kinase; myosin is dephosphorylated by myosin light-chain phosphatase and the muscle relaxes. Recent evide nce suggests that other signal-transduction pathways can modulate the contractile state of a smooth-muscle cell by affecting specific steps in the myosin phosphorylation-dephosphorylation mechanism.