Snapshot A 46-year-old woman with a past medical history of systemic lupus erythematosus comes to the physician’s office for discolored fingers that occur in the winter. She reports that this usually is not very painful and only happens when it is cold outside. Her fingers usually turn white first, then blue, and finally become pink with re-warming. She is prescribed calcium-channel blockers. When asked why this happens, her physician tells her that this is most likely due to an exaggerated vasospasm of the smooth muscles in her blood vessels in response to cold temperatures. (Raynaud phenomenon) Introduction Smooth muscle has bundles of contractile proteins that criss-cross in a lattice across the cell Smooth muscle is able to maintain a high force of contraction for less ATP than skeletal muscles Contraction Rise in intracellular calcium ions (Ca2+) login to view 5 more bullets Ca2+ bind to calmodulin, which induces conformational change login to view 2 more bullets Termination of Contraction Nitric oxide login to view 2 more bullets Myosin light chain phosphatase (MLCP) login to view 1 more bullet Sarcoplasmic reticulum Ca2+ ATPase 2 (SERCA2) login to view 1 more bullet