Question

Draw a schematic of the mechanism of the Latch Bridge for sustained smooth muscle contraction.

Answer 1

At the beginning of the contraction of the smooth muscle the calcium levels inside the smooth muscle cell are quite high, leading to lots of phosphorylation of the myosin light chain.   As the contraction lasts longer, the intracellular calcium levels begin to decline due to the action of the calcium pumps. As the calcium levels drop, the calcium begins to leave the calmodulin, leading to inactivation of the mysin light chain kinase MLCK and activation of the myosin light chain phosphatase.   The phosphatase removes the phosphate from some of the light chains. Those cross-bridges that are dephosphorylated immediately begin to detach.Well, something interesting occurs here - once the actin-myosin complex goes through the powerstroke, they remain stuck (or latched) together for quite some time. This is because the actin-myosin complex that has dephosphorylated light chains has a very low affinity for the ATP that must come along to release the two of them. Until that ATP comes along, the two remain in the latched position and continue to generate tension. The following diagram illustrates this-