Question: where do smooth cells get most of their calcium to stimulate contraction

title: Smooth muscle tissue
text: The phosphorylation of the light chains by MLCK is countered by a myosin light - chain phosphatase , which dephosphorylates the MLC myosin light chains and thereby inhibits contraction . Other signaling pathways have also been implicated in the regulation actin and myosin dynamics . In general , the relaxation of smooth muscle is by cell - signaling pathways that increase the myosin phosphatase activity , decrease the intracellular calcium levels , hyperpolarize the smooth muscle , and / or regulate actin and myosin muscle can be mediated by the endothelium - derived relaxing factor - nitric oxide , endothelial derived hyperpolarizing factor ( either an endogenous cannabinoid , cytochrome P450 metabolite , or hydrogen peroxide ) , or prostacyclin ( PGI2 ) . Nitric oxide and PGI2 stimulate soluble guanylate cyclase and membrane bound adenylate cyclase , respectively . The cyclic nucleotides ( cGMP and cAMP ) produced by these cyclases activate Protein Kinase G and Protein Kinase A and phosphorylate a number of proteins . The phosphorylation events lead to a decrease in intracellular calcium ( inhibit L type Calcium channels , inhibits IP3 receptor channels , stimulates sarcoplasmic reticulum Calcium pump ATPase ) , a decrease in the 20kd myosin light chain phosphorylation by altering calcium sensitization and increasing myosin light chain phosphatase activity , a stimulation of calcium sensitive potassium channels which hyperpolarize the cell , and the phosphorylation of amino acid residue serine 16 on the small heat shock protein ( hsp20 ) by Protein Kinases A and G . The phosphorylation of hsp20 appears to alter actin and focal adhesion dynamics and actin - myosin interaction , and recent evidence indicates that hsp20 binding to 14 - 3 - 3 protein is involved in this process . An alternative hypothesis is that phosphorylated Hsp20 may also alter the affinity of phosphorylated myosin with actin and inhibit contractility by interfering with crossbridge formation . The endothelium derived hyperpolarizing factor stimulates calcium sensitive potassium channels and / or ATP sensitive potassium channels and stimulate potassium efflux which hyperpolarizes the cell and produces relaxation .